CI: Twisted reactor unit test is flaky on macOS arm PyPy

[dkropachev]

Summary

Test wheels building / Build wheels for macos-arm on macos-14 failed on pp311-macosx_arm64 with a Twisted unit test race.

Failing run/job:
https://github.com/scylladb/python-driver/actions/runs/28405982626/job/84168473960?pr=895

Failure

Failed test:

tests/unit/io/test_twistedreactor.py::TestTwistedConnection::test_connection_initialization

Assertion failure:

Expected: run(installSignalHandlers=False)
Actual: not called.

The test creates a TwistedConnection, which calls TwistedLoop.maybe_start(). maybe_start() starts the reactor on a background thread:

self._thread = Thread(target=reactor.run, kwargs={'installSignalHandlers': False})
self._thread.start()

The test immediately asserts that mocked reactor.run was called. Thread.start() can return before the target function runs, so on slower/different scheduling, especially PyPy on macOS arm, the assertion can run first.

Notes

This appears flaky rather than caused by PR #895. The PR only removed an unused import from tests/unit/io/test_twistedreactor.py; it did not change Twisted reactor behavior.

In the same macOS arm job, CPython 3.10 through 3.14 passed. The failure occurred only on PyPy 3.11.

Possible fixes

• Wait/poll for the mocked reactor.run call before asserting.
• Or avoid asserting immediate background-thread execution; mock Thread and assert target/kwargs/start behavior instead.
• Ensure teardown does not leave reactor/thread state that can affect later tests.