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

[sobolevn]

(cherry picked from commit bfc1d25)

Extra changes:

• I modified added_f = functools.partial(str_converter, **kwargs) # type: ignore[arg-type] because there was an error (basically, we can ignore functools.partial errors most of the time, because they are very hard to get right)
• Synced requirements-dev.txt with main

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

[sobolevn]

Oh, libregrtest is not even checked on 3.12 🤦

[ambv]

3.12 is entering security-only fixes mode today.