Update mypy to latest version and support mypy --strict

[AMDmi3]

Long story short

Following discussion in #3868, here's an issue to track a progress of cleaning up errors reported by newer mypy and/or mypy in strict mode.

Expected behaviour

aiohttp is clean with latest mypy (0.720 ATOW) in --strict mode.

Actual behaviour

There are some errors after updating mypy, and a lot of errors after enabling strict mode.

Steps to reproduce

Run mypy --strict aiohttp with latest mypy

Your environment

FreeBSD 12.0 amd64, python 3.7, mypy 0.720

[AMDmi3]

(I'll try to fix at least a part of this)