Add support for Python 3.10 and 3.11, drop EOL 3.6

[hugovk]

Test 3.10 and 3.11, and add Trove classifiers.

Drop 3.6, EOL since 2021-09-03.

Also bump various versions:

• GitHub Actions
• docs dependencies
• dev dependencies

[TheTripleV]

Can this be closed in favor of #70 then?

[hugovk]

Yep, closing in favour of #70.