Drop support for EOL Python 3.6

[hugovk]

We can drop Python 3.6 in the next release because it is EOL and no longer receiving security updates (or any updates) from the core Python team.

cycle	latest	release	eol
3.10	3.10.1	2021-10-04	2026-10-04
3.9	3.9.9	2020-10-05	2025-10-05
3.8	3.8.12	2019-10-14	2024-10-14
3.7	3.7.12	2018-06-27	2023-06-27
3.6	3.6.15	2016-12-23	2021-12-23

https://endoflife.date/python

This PR also removes some CI config boilerplate by using pip caching that is now built into https://github.com/actions/setup-python.

[codecov]

Codecov Report

Merging #513 (b81cd32) into master (91beb35) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #513   +/-   ##
=======================================
  Coverage   90.82%   90.82%           
=======================================
  Files          28       28           
  Lines        2660     2660           
=======================================
  Hits         2416     2416           
  Misses        244      244           

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[claudep]

Thanks, a good start for 2022!!