libregrtest: Improve type hints; explore running mypy in CI

[AlexWaygood]

Feature or enhancement

Proposal:

The code in Lib/test/libregrtest already has a smattering of type hints in it, but doesn't currently pass type checking. @vstinner, the primary maintainer of libregrtest, expressed interest to me in seeing whether we could run mypy on libregrtest in CI.

This issue is for tracking progress towards that.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Linked PRs

• gh-109413: libregrtest: Add and improve type annotations #109405
• gh-109413: Fix some trivial mypy nitpicks in libregrtest #109454
• gh-109413: Add a custom script for running mypy on libregrtest #109464
• gh-109413 Improve the mypy config file for libregrtest #109518
• gh-109413: Run mypy on libregrtest in CI #112558
• gh-109413: libregrtest: enable mypy's --strict-optional check on most files #112586
• gh-109413: regrtest: add WorkerRunTests class #112588
• [3.12] gh-109413: regrtest: add WorkerRunTests class (GH-112588) #112593
• [3.11] gh-109413: regrtest: add WorkerRunTests class (GH-112588) #112594
• [3.12] gh-109413: libregrtest: enable mypy's --strict-optional check on most files (#112586) #112602
• [3.11] [3.12] gh-109413: libregrtest: enable mypy's --strict-optional check on most files (GH-112586) (GH-112602) #112603
• [3.12] gh-109413: libregrtest: Backport .py-file changes from #112558 (#112558) #112605
• [3.11] [3.12] gh-109413: libregrtest: Backport .py-file changes from GH-112558 (GH-112605) #112607
• gh-109413: Add more type hints to libregrtest #126352
• [3.13] gh-109413: Add more type hints to libregrtest (GH-126352) #126388
• [3.12] gh-109413: Add more type hints to libregrtest (GH-126352) #126389
• gh-109413: Enable strict_optional for libregrtest/main.py #126394
• [3.13] gh-109413: Enable strict_optional for libregrtest/main.py (GH-126394) #126802
• gh-109413: Enable strict_optional = true for libregrtest/run_workers #126855
• [3.13] gh-109413: Enable strict_optional = true for libregrtest/run_workers (GH-126855) #126967
• gh-109413: Enable mypy's disallow_any_generics setting when checking libregrtest #127033
• [3.13] gh-109413: Enable mypy's disallow_any_generics setting when checking libregrtest (GH-127033) #127034
• gh-109413: Fix libregrtest get_running() #127153
• [3.13] gh-109413: Fix libregrtest get_running() (GH-127153) #127157

[AlexWaygood]

We currently do not have any projects inside the Lib/ directory that are mypy-checked in CI: currently, all the mypy-checked projects we have in CPython live in the Tools/. If we want to actually run mypy on libregrtest in CI, I think it should be discussed on Discourse first, to check if anybody has any objections to this idea.