PYTHON-5387 Add beta Python 3.15

[aclark4life]

Per uv docs:

• "Python pre-releases will be used if there is no other available installation matching the request."

PYTHON-5387

Changes in this PR

Test Plan

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s).

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?

[Copilot]

Pull request overview

Updates the GitHub Actions Python test workflow to start exercising the test suite against the Python 3.15 beta via uv’s Python installer/version resolution.

Changes:

• Extend the CI test matrix in test-python.yml to include Python 3.15 alongside existing CPython/PyPy entries.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[blink1073]

Oooh interesting, these failures failures seem worth addressing.

FAILED test/test_fork.py::TestFork::test_lock_client - DeprecationWarning: This process (pid=3147) is multi-threaded, use of fork() may lead to deadlocks in the child.
FAILED test/test_fork.py::TestFork::test_lock_object_id - DeprecationWarning: This process (pid=3147) is multi-threaded, use of fork() may lead to deadlocks in the child.
FAILED test/test_fork.py::TestFork::test_topology_reset - DeprecationWarning: This process (pid=3147) is multi-threaded, use of fork() may lead to deadlocks in the child.

I think we can safely ignore those warnings unless the same client is used across threads, assuming the monitor threads get re-created during the fork.