Raise PackageNotFoundError for a non-zip stream, not BadZipFile

[eeshsaxena]

Opening a document from a path that isn't a package raises PackageNotFoundError, but opening one from a stream that isn't a zip leaks a bare zipfile.BadZipFile:

import io, docx
docx.Document(io.BytesIO(b"not a docx"))
# zipfile.BadZipFile: File is not a zip file

PhysPkgReader.__new__ checks is_zipfile() for a path and raises PackageNotFoundError when it isn't a package, but for a stream it hands the file straight to the zip reader ("assume it's a stream and pass it to Zip reader to sort out"). The zip reader never sorted out the not-a-zip case, so callers catching PackageNotFoundError (the documented error for a bad package) miss the stream case.

This catches BadZipFile in _ZipPkgReader and re-raises PackageNotFoundError, so a bad stream and a bad path fail the same way. Valid packages are unaffected (verified a normal open/save/read round-trip still works). Added a test mirroring the existing not-a-package-path test.