Python 3.14 support

[namachan10777]

The current pfio version cannot be used with 3.14 because it relies on changes or removed features introduced in 3.14. We will address these current 3.14 incompatibilities one by one before implementing full 3.14 support.

[namachan10777]

• Call as_uri via pathlib.Path instance in compatibility test #371
• Migrate is_reserved usage to os.path.isreserved on Python 3.14+ #373
• Adapt PurePath.relative_to to Python 3.14 #374
• Make multiprocessing tests forkserver-safe #375
• Create PR for 3.14 support

[kuenishi]

Please make sure that you mark at least one milestone for every issue you're going to work on.

[namachan10777]

This is completed by #376