gh-109413: libregrtest: Add and improve type annotations

[AlexWaygood]

@vstinner, this is my attempt to create a PR with only "non-controversial" type hint changes, as you requested in #109382 (review). I'm not fully sure I understand exactly which changes you consider to be controversial, though, so apologies if there are still some changes here that you feel should be deferred to a future PR :-)

(For example: is the addition of a mypy config file in this PR okay? If not, I'm not sure how to test locally the changes I'm making here.)

This PR gets us down to 20 mypy errors if you cd into Lib/test and then run mypy --config-file libregrtest/mypy.ini.

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

[vstinner]

I am not sure that the config file without associate script is useful in the PR.

Here is a first review.

[AlexWaygood]

I am not sure that the config file without associate script is useful in the PR.

It can be used for local testing. You can cd into Lib/test, and then run mypy --config-file libregrtest/mypy.ini.

Without the config file, I don't know how to verify that the changes I'm making in this PR are correct. If I don't know whether the annotations I'm adding are correct or not, then I don't think there's much point in adding them.

[AlexWaygood]

It can be used for local testing. You can cd into Lib/test, and then run mypy --config-file libregrtest/mypy.ini.

Without the config file, I don't know how to verify that the changes I'm making in this PR are correct. If I don't know whether the annotations I'm adding are correct or not, then I don't think there's much point in adding them.

Though I suppose I can keep the config around locally, but not commit it in this PR, if you prefer.

[vstinner]

LGTM, but please keep self.testsuite_xml: list[str] = [] bug on purpose for now. The code smells, maybe something is wrong here.

[AlexWaygood]

Thanks for the review @vstinner!