Upgrade Python type checker mypy

[cclauss]

Summary

Unblocks #3653 which is currently failing on the command mypy httpx tests.

• Bump the python-packages group across 1 directory with 11 updates #3653
• https://github.com/python/mypy/tags

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.