2/X Fix check_untyped_defs = True mypy errors

[bluetech]

Fixes for some more check_untyped_defs errors from mypy.

The first commit required a small refactoring (see commit message).

Continuation of #5673.

[bluetech]

The CI failed two times with

+ bash /dev/fd/63 -Z -X gcov -X coveragepy -X search -X xcode -X gcovout -X fix -f coverage.xml
++ curl -s https://codecov.io/bash
bash: /dev/fd/63: No such file or directory

coming from

pytest/scripts/report-coverage.sh

Line 16 in c8cf748

bash <(curl -s https://codecov.io/bash) -Z -X gcov -X coveragepy -X search -X xcode -X gcovout -X fix -f coverage.xml

I don't think it is related to the changes.

[The-Compiler]

Note that mypy 0.740 substantially improved check_untyped_defs - in my project, I got 469 new errors 😆

Could you check how things look with pytest and whether you're still willing to invest the effort to get it clean of errors?

[bluetech]

Could you check how things look with pytest

I tried 0.740 on pytest now -- it adds a dozen unrelated errors, and only a few new check_untyped_defs errors. So not too worrisome. I'll send a PR to update to 0.740 now (hopefully).

and whether you're still willing to invest the effort to get it clean of errors?

Sure. I am trying to keep the PRs reasonably small, and only have one in-flight PR at a time, to not overwhelm anyone. However, there doesn't seem to be any volunteers to review these changes (understandably), so it's going to take a while.

BTW, I rebased this PR now.

[blueyed]

LGTM. Can be changed after mypy v0.740 is merged.
And see #6039.

[blueyed]

Thanks!