[3.13] gh-109413: Add more type hints to libregrtest (GH-126352)

[miss-islington]

(cherry picked from commit bfc1d25)

Co-authored-by: sobolevn [email protected]

• Issue: libregrtest: Improve type hints; explore running mypy in CI #109413

[Yhg1s]

This backport seems to have some test failures and merge conflicts that need looking at.

[sobolevn]

Fixing! Thanks for the ping.

[vstinner]

@sobolevn: There are conflicts again, and many CI tests are failing.

  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/libregrtest/runtests.py", line 109, in RunTests
    def create_worker_runtests(self, **override) -> WorkerRunTests:
                                                    ^^^^^^^^^^^^^^
NameError: name 'WorkerRunTests' is not defined

[sobolevn]

Done, should be fixed now. Sorry for the long delay.