Add support for python 3.10

[kclowes]

@cburgdorf how do I run the tests? I vaguely remember doing this before but can't remember how 🤔

[cburgdorf]

It should be make test_all

https://github.com/ethereum/blake2b-py/blob/473e9d5d6d39c40e1713c343937505c438287b46/Makefile#L26-L27

As far as cutting a release goes, it should go out automatically (including pushing to pypi) when you push a tag into the repo.

[kclowes]

Got it, thanks! This is ready for review when you get a chance!

[cburgdorf]

Looks good!