feat: support standard library pathlib.Path for Document open/save

[SamYue1]

Problem

python-docx currently only accepts str or IO[bytes] for the Document() factory function and Document.save(). Passing a pathlib.Path object — the standard way to represent file paths in modern Python — either causes type-checking errors (e.g. with pyright, mypy) or, in the case of PhysPkgReader, bypasses the _DirPkgReader code path because isinstance(pkg_file, str) does not match Path objects.

Solution

• Update type annotations from str | IO[bytes] to str | os.PathLike[str] | IO[bytes] across the call chain: Document(), Document.save(), DocumentPart.save(), OpcPackage.open(), OpcPackage.save().
• Convert os.PathLike → str at each entry point using os.fspath() so that downstream code (which expects str) continues to work unchanged.
• Fix PhysPkgReader.__new__() to check isinstance(pkg_file, (str, os.PathLike)) instead of isinstance(pkg_file, str), so Path objects are correctly routed to _DirPkgReader or _ZipPkgReader.

Changes

• src/docx/api.py — type annotation + conversion in Document()
• src/docx/document.py — type annotation + conversion in Document.save()
• src/docx/parts/document.py — type annotation + conversion in DocumentPart.save()
• src/docx/opc/package.py — type annotation + conversion in OpcPackage.open() / save()
• src/docx/opc/phys_pkg.py — fix isinstance check in PhysPkgReader.__new__()
• tests/test_api.py, tests/test_document.py, tests/parts/test_document.py, tests/opc/test_phys_pkg.py — add unit tests for Path support

Backward Compatibility

Fully backward compatible. All 90+ existing tests pass without modification.