Add parent_mode to Path.mkdir on Python 3.15 - #259

What changed

Python 3.15 added a keyword-only parent_mode parameter to pathlib.Path.mkdir():

pathlib.Path.mkdir(self, mode=0o777, parents=False, exist_ok=False, *, parent_mode=None)

aiofiles.pathlib.Path documents that its methods follow the pathlib API of the running interpreter, and 3.15 is covered by CI/tox/changelog, but its hand-written mkdir() still had the pre-3.15 four-argument signature. Calling mkdir(parents=True, parent_mode=0o700) on 3.15 therefore raised:

TypeError: Path.mkdir() got an unexpected keyword argument 'parent_mode'

This adds a version-gated keyword-only parent_mode for Python 3.15+, forwarded to the wrapped pathlib.Path.mkdir. The signature on Python 3.9-3.14 is unchanged (unconditional forwarding would itself raise TypeError there).

Fixes #258.

Tests

• New test_mkdir_parent_mode (3.15+ only) asserts that non-leaf parents receive parent_mode and the leaf receives mode, accounting for the process umask on POSIX.
• Existing test_mkdir_and_rmdir continues to cover the ordinary parents/exist_ok paths.
• Python 3.15.0: pytest -q -> 285 passed, 8 skipped; pytest -q tests/test_pathlib.py -k mkdir -> 2 passed.
• Python 3.14.7: ordinary mkdir(parents=True) works, and parent_mode is rejected as expected for that interpreter.
• ruff format --check src tests and ruff check src tests pass.

Notes

• aiofiles.pathlib is new in the unreleased 25.2.0, so this is a pre-release completeness fix rather than a regression in a released version.
• The changelog entry is under 25.2.0 and references aiofiles.pathlib.Path.mkdir does not accept Python 3.15's parent_mode #258.

Prepared with Claude Code, an LLM agent operating the BlueX888 account.