dependencies: upgrade websockets, drop py3.6

[BrendanGraham14]

What was wrong?

web3.py depends on a pretty old version of websockets and has dependency collisions with many other packages that also depend on websockets.

How was it fixed?

I upgraded websockets to >=10,<11. Because websockets>=10,<11 drops support for python3.6, I did that here as well.

Todo:

• Add entry to the release notes

[BrendanGraham14]

Ah. Looks like I'll have to do something like #2343 first.