Drops python3.6 support

[sobolevn]

I am pretty sure that I haven't covered all possible cases in this PR at the moment.
But, I hope that this is a good start 🙂

Any feedback is welcome!

Closes #3174

[Zac-HD]

Looks good! I'm looking forward to merging this on the 23rd 😁

I've left some comments from looking through the diff, primarily about missed-refactorings around importlib.resources and the types defined in compat.py, and pushed a commit based on my manual audit of git grep -nE "[pP]y(thon)? ?3|sys\.version" -- . ':!*/changes.rst' results.

At some point I'll also want to check everything that we configure coverage to ignore, but that's pretty low priority.

[sobolevn]

Done, fixed all the things you've mentioned! Thanks a lot, @Zac-HD!

[Zac-HD]

✅

I hereby declare this finished! On EOL day, 2021-12-23, I'll approve and merge 🚀

[Zac-HD]

If you're interested in putting some typing hacks back in support for new typing features in 3.9/3.10, we have #2978 and #2979 and arguably also #3150 😉

[Zac-HD]

🎉

[sobolevn]

Congrats! 🚀

Unrelated, but, mypy 0.930 was released, let me know if there are any regressions / new things to fix in hypothesis! 👍

[Zac-HD]

I just rebuilt #3197 with the new versions of mypy, black, and shed! Moved some type-ignores around but it's a pretty small change 😎

Best new things to fix are, well, pick an issue! The typing ones in particular might suit you?

[sobolevn]

#2978 is still in my backlog 😎