Python 3.13: expect DeprecationWarning for pathlib.PurePath.is_reserved()

[musicinmybrain]

Changes

This is a partial fix for #737.

On Python 3.13 and later, test_is_reserved now expects that pathlib.PurePath.is_reserved() produces a DeprecationWarning.

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) added which would fail without your patch N/A
• You've updated the documentation (in docs/, in case of behavior changes or new
  features) N/A
• You've added a new changelog entry (in docs/versionhistory.rst).

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
these instructions.

[agronholm]

Sorry, but I already had local changes ready that tackle not only this issue, but also the issue of missing 3.13 Path features.