Run format and mypy on Python 3.8

[Kludex]

I don't understand how master is passing...

Master is passing because it passed 6 months ago, but dependencies changed, and something else also changed on the Python action, not sure what... 🤷

Checklist

• Pin mypy.

[Kludex]

@sethmlarson Can I bother you? 👀

There are also the following that can be merged after this:

• Don't set connection close headers when they are already set on the Response #171
• Fix _SWITCH_UPGRADE without proposal error message #168
• Remove tests folder from wheels #159

It would be cool to make a release here, it's been some time.

[sethmlarson]

LGTM, thank you!