Backport #5457

[Dreamsorcerer]

Improve Mypy coverage

I've skipped checking tests on this one as there are a lot of type errors coming through.

[lgtm-com]

This pull request fixes 1 alert when merging ac739e5 into 9a09d1b - view on LGTM.com

fixed alerts:

• 1 for Wrong number of arguments in a call

[webknjaz]

Looks like this needs patching...

[Dreamsorcerer]

Problem is already in the 3.8 branch (see failed tests on last commit to 3.8).

It looks like the CIMultiDictProxy[str] syntax should work because of this code:
https://github.com/aio-libs/multidict/blob/master/multidict/_abc.py#L7

Is it possible that the functionality is missing from the C version?
https://github.com/aio-libs/multidict/blob/master/multidict/_multidict.c

Or any other ideas @webknjaz?

[Dreamsorcerer]

Problematic annotation was added in: 383016b#diff-197153342549adb4b25580f8b842876841eec42dc48ae12cb72ba2ae4f7ced10R79

I can annotate it another way if needed, but I'd expect it to work if the class supports __getitem__

[webknjaz]

Dunno, maybe quote it?

[derlih]

I've created the PR #5448 to fix the problem on a CI. But I don't know how to solve the mypy issue there :-(

[lgtm-com]

This pull request fixes 1 alert when merging 9cbcb41 into 30f90c5 - view on LGTM.com

fixed alerts:

• 1 for Wrong number of arguments in a call