Looks like that worked! I can also reproduce the problem (at least for test_norm_callback) locally with an update of Python 3.13.11 to 3.13.12.

I don't have a dev-Python setup, but I think @tacaswell does; maybe you can run a bisect there?

A quick perusal through the changelog suggests:

gh-142651: unittest.mock: fix a thread safety issue where Mock.call_count may return inaccurate values when the mock is called concurrently from multiple threads.

which was applied to both 3.14.3 and 3.13.12.

A comment there suggests that we should not be modifying call_count ourselves, but instead using reset_mock.

Also note that Azure had the same problem, since I didn't change that build. Surprisingly no timeouts involved though.

Closing in favour of #31153; this was enough to point me in the right direction.