implement flake8-type-checking

[half-duplex]

Description

Tin. See #1765

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

[half-duplex]

Had to restrict it to python 3.8+ since flake8-type-checking doesn't support 3.7...

[dgw]

Think I threw it on the todo pile in #1765 instead of just doing it immediately because there's no support for 3.7… But there's actually nothing that breaks if someone runs style checks on 3.7, only the potential for missing TC error codes that CI will flag. And probably no one should be running their main development on anything older than 3.8 or 3.9… so I declare my past self silly.

Can you exclude/ignore the TC200 error series? We want to use delayed evaluation (it's why our files use from __future__ import annotations), not forward references. Might also want type-checking-exempt-modules = typing, which would make the places where from typing appears both inside and outside the TYPE_CHECKING block unnecessary.

(My present self is also silly, just for different reasons.)

[dgw]

Cool. Time to ask for review from the one with actual experience using mypy.

[Exirel]

That's interesting. Maybe it's also time to officially add mypy to the mix then?

[dgw]

Could be, if it doesn't generate any errors after merging with master. Whatcha think about this otherwise?

[Exirel]

I'm OK with that as long as make mypy works.