build: drop Python 3.6

[staticdev]

Closes #477

[staticdev]

@Bogdanp could you review my PR please?

[Bogdanp]

Thanks. This looks fine, but I'm not in a hurry to remove 3.6 yet.

[staticdev]

Thanks. This looks fine, but I'm not in a hurry to remove 3.6 yet.

@Bogdanp My idea is to contribute to dramatiq type checking capabilities, but python 3.6 offers very poor support and many problems with type checking. This is the main reason of this PR. From 3.7, things are much smoother with from __future__ import annotations. Please consider that, since most modern libraries such as dramatiq have fully enabled annotations for the applications using it.

Also as I mentioned in #477, 3.6 EOL was on 23 Dec 2021, we are almost done with python 3.11 ;)

[Bogdanp]

Feel free to work on that with dropping 3.6 in mind, but I think we'll wait at least another 6 months before actually dropping 3.6, like we did 3.5 (around 1 year after it was EOL'd). Users may move more slowly than Python core does and I value stability more than I value new features.