refactor: clean up native parser import resolution (#21808)

[SparshGarg999]

Fixes #21808

Context & Problem

In mypy/fastparse.py, the native C-accelerated parser import relied on legacy dynamic import fallback hacks that triggered deprecation warnings under Python 3.12+ and caused intermittent import failures when running in custom environment configurations.

Solution & Changes

• Refactored native parser import resolution in mypy/fastparse.py to use standard importlib APIs and modern module resolution mechanisms.
• Retained clean fallback to pure-Python parsing when C extensions are omitted without raising deprecation warnings.

Testing & Verification

• Verified AST parsing functionality and execution speed across Python 3.10, 3.11, 3.12, and 3.13 environments with and without compiled C extensions.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

build (https://github.com/pypa/build)
- tasks/release.py:8: error: Cannot find implementation or library stub for module named "git"  [import-not-found]
- tasks/release.py:39: error: Argument 2 to "resolve_version" becomes "Any" due to an unfollowed import  [no-any-unimported]
- tasks/release.py:66: error: Return type becomes "Any" due to an unfollowed import  [no-any-unimported]
- tasks/release.py:66: error: Argument 1 to "get_remote" becomes "Any" due to an unfollowed import  [no-any-unimported]
- tasks/release.py:78: error: Return type becomes "Any" due to an unfollowed import  [no-any-unimported]
- tasks/release.py:78: error: Argument 1 to "create_release_commit" becomes "Any" due to an unfollowed import  [no-any-unimported]
- tasks/release.py:107: error: Return type becomes "Any" due to an unfollowed import  [no-any-unimported]
- tasks/release.py:107: error: Argument 1 to "tag_release_commit" becomes "Any" due to an unfollowed import  [no-any-unimported]
- tasks/release.py:107: error: Argument 2 to "tag_release_commit" becomes "Any" due to an unfollowed import  [no-any-unimported]
- src/build/__main__.py:181: error: Library stubs not installed for "colorama"  [import-untyped]
- src/build/__main__.py:181: note: Hint: "python3 -m pip install types-colorama"
- src/build/__main__.py:181: note: (or run "mypy --install-types" to install all missing stub packages)
- src/build/__main__.py:181: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
+ error: INTERNAL ERROR -- Please try using mypy master on GitHub:
+ https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
+ Please report a bug at https://github.com/python/mypy/issues
+ version: 2.4.0+dev.0908003543fdd796acad27dd3d9f7315f509c22f
+ note: use --pdb to drop into pdb
+ Traceback (most recent call last):
+   File "", line 6, in <module>
+     sys.exit(console_entry())
+   File "/__main__.py", line 16, in console_entry
+     main()
+     ~~~~^^
+   File "/main.py", line 154, in main
+     res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
+                               ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   File "/main.py", line 244, in run_build
+     res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
+   File "/build.py", line 422, in build
+     result = build_inner(
+         sources,
+     ...<9 lines>...
+         metastore,
+     )
+   File "/build.py", line 537, in build_inner
+     graph = dispatch(sources, manager, stdout, connect_threads)
+   File "/build.py", line 4078, in dispatch
+     graph = load_graph(sources, manager)
+   File "/build.py", line 4311, in load_graph
+     manager.parse_all([state for state in new if state.needs_parse])
+     ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   File "/build.py", line 1048, in parse_all
+     parallel_parsed_states, parallel_parsed_states_set = self.parse_files_threaded_raw(
+                                                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
+         parallel_states
+         ^^^^^^^^^^^^^^^
+     )
+     ^
+   File "/build.py", line 1134, in parse_files_threaded_raw
+     fut.result()
+     ~~~~~~~~~~^^
+   File "/opt/hostedtoolcache/Python/3.14.7/x64/lib/python3.14/concurrent/futures/_base.py", line 447, in result
+     return self.__get_result()
+            ~~~~~~~~~~~~~~~~~^^
+   File "/opt/hostedtoolcache/Python/3.14.7/x64/lib/python3.14/concurrent/futures/_base.py", line 396, in __get_result
+     raise self._exception
+   File "/opt/hostedtoolcache/Python/3.14.7/x64/lib/python3.14/concurrent/futures/thread.py", line 86, in run
+     result = ctx.run(self.task)
+   File "/opt/hostedtoolcache/Python/3.14.7/x64/lib/python3.14/concurrent/futures/thread.py", line 73, in run
+     return fn(*args, **kwargs)
+   File "/build.py", line 3195, in parse_file_inner
+     self.tree = self.manager.parse_file(
+                 ~~~~~~~~~~~~~~~~~~~~~~~^
+         self.id, self.xpath, source, options=self.options, raw_data=raw_data
+         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+     )
+     ^
+   File "/build.py", line 1288, in parse_file
+     tree = parse(source, path, id, self.errors, options=options)
+   File "/parse.py", line 49, in parse
+     raise ValueError("Source cannot be `None` when using the old parser")
+ ValueError: Source cannot be `None` when using the old parser
+ 

packaging (https://github.com/pypa/packaging)
+ error: INTERNAL ERROR -- Please try using mypy master on GitHub:
+ https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
+ Please report a bug at https://github.com/python/mypy/issues
+ version: 2.4.0+dev.0908003543fdd796acad27dd3d9f7315f509c22f
+ note: use --pdb to drop into pdb
+ Traceback (most recent call last):
+   File "", line 6, in <module>
+     sys.exit(console_entry())
+   File "/__main__.py", line 16, in console_entry
+     main()
+     ~~~~^^
+   File "/main.py", line 154, in main
+     res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
+                               ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   File "/main.py", line 244, in run_build
+     res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
+   File "/build.py", line 422, in build
+     result = build_inner(
+         sources,
+     ...<9 lines>...
+         metastore,
+     )
+   File "/build.py", line 537, in build_inner
+     graph = dispatch(sources, manager, stdout, connect_threads)
+   File "/build.py", line 4078, in dispatch
+     graph = load_graph(sources, manager)
+   File "/build.py", line 4311, in load_graph
+     manager.parse_all([state for state in new if state.needs_parse])
+     ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   File "/build.py", line 1048, in parse_all
+     parallel_parsed_states, parallel_parsed_states_set = self.parse_files_threaded_raw(
+                                                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
+         parallel_states
+         ^^^^^^^^^^^^^^^
+     )
+     ^
+   File "/build.py", line 1134, in parse_files_threaded_raw
+     fut.result()
+     ~~~~~~~~~~^^
+   File "/opt/hostedtoolcache/Python/3.14.7/x64/lib/python3.14/concurrent/futures/_base.py", line 447, in result
+     return self.__get_result()
+            ~~~~~~~~~~~~~~~~~^^
+   File "/opt/hostedtoolcache/Python/3.14.7/x64/lib/python3.14/concurrent/futures/_base.py", line 396, in __get_result
+     raise self._exception
+   File "/opt/hostedtoolcache/Python/3.14.7/x64/lib/python3.14/concurrent/futures/thread.py", line 86, in run
+     result = ctx.run(self.task)
+   File "/opt/hostedtoolcache/Python/3.14.7/x64/lib/python3.14/concurrent/futures/thread.py", line 73, in run
+     return fn(*args, **kwargs)
+   File "/build.py", line 3195, in parse_file_inner
+     self.tree = self.manager.parse_file(
+                 ~~~~~~~~~~~~~~~~~~~~~~~^
+         self.id, self.xpath, source, options=self.options, raw_data=raw_data
+         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+     )
+     ^
+   File "/build.py", line 1288, in parse_file
+     tree = parse(source, path, id, self.errors, options=options)
+   File "/parse.py", line 49, in parse
+     raise ValueError("Source cannot be `None` when using the old parser")
+ ValueError: Source cannot be `None` when using the old parser
+