PYTHON-5874 Fix test_fork.py failures on Python 3.15 due to fork() DeprecationWarning

[blink1073]

PYTHON-5874

Python 3.12+ emits a DeprecationWarning when os.fork() is called in a multi-threaded process, which the test suite's filterwarnings = ["error"] turns into a failure for the test_fork.py tests. These tests intentionally fork a running client whose register_at_fork handlers reset its locks in the child, so the warning is benign; suppress it in the self.fork helper and remove the 3.15 skipIf stopgap.

Changes in this PR

• Suppress the Python 3.12+ os.fork() DeprecationWarning inside the self.fork test helper, scoped to that specific warning.
• Removed the skipIf that skipped test_fork.py on Python 3.15+ (stopgap from PYTHON-5837) so the tests run on 3.15+.

Test Plan

• test/test_fork.py on Python 3.15.0rc1: 3/3 pass (previously 2 failed with the DeprecationWarning).
• test/test_fork.py on Python 3.13: 3/3 pass (no regression).
• ruff, ruff-format, and mypy clean on the changed files.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)? — not necessary; test-only change with no driver behavior change.
• Is there test coverage? — the fix is to existing tests; no new coverage needed.
• Is any followup work tracked in a JIRA ticket? If so, add link(s). — documentation note tracked on PYTHON-5874 (Documentation Changes = Needed); docs live externally on mongodb.com.

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]

🟢 Approval recommended

The warning suppression is narrowly scoped to the intentional fork helper and the removed 3.15+ skip is justified by that targeted mitigation.

Pull request overview

This PR updates the PyMongo test harness to keep test_fork.py running on Python 3.12+ (including 3.15) by suppressing the new os.fork() multi-threaded DeprecationWarning only at the point where the test suite intentionally forks, rather than skipping the tests.

Changes:

• Suppress Python 3.12+ os.fork() multi-thread DeprecationWarning inside the shared IntegrationTest.fork helper (scoped to just proc.start()).
• Remove the Python 3.15+ skipIf stopgap from test/test_fork.py so the fork tests execute on 3.15+.
• Add/adjust comments explaining why the warning is benign in this test scenario (PYTHON-5874).

File summaries

File	Description
test/test_fork.py	Removes the 3.15+ skip and documents that the fork helper suppresses the warning for intentional forks.
test/asynchronous/init.py	Adds a localized warnings filter around proc.start() in the fork helper to ignore the fork DeprecationWarning.
test/init.py	Mirrors the same localized warnings suppression in the synchronous test helper.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 2
• Review effort level: Lite

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[codeowners-service-app]

Assigned aclark4life for team dbx-python because NoahStapp is out of office.

[aclark4life]

LGTM