Fix ast warnings for Python3.12

[sobolevn]

Warnings before: #15330 (comment)
Warnings after:

(.venv312) ~/Desktop/mypy  master ✗                                                       
» python -m mypy ex.py
Success: no issues found in 1 source file

Zero :)

[JelleZijlstra]

Seems like there's more to be done. We're getting a None here:

  File "/home/runner/work/mypy/mypy/mypy/fastparse.py", line 1960, in _extract_argument_name
    return n.s.strip()

The mypy-primer hit also likely indicates a bug. It's something with type comments.

[JelleZijlstra]

We dropped support for 3.7 by now, so this can probably be simplified; at least there's a big merge conflict now.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅