Drop support for Python 3.6

[hugovk]

Python 3.6 is (almost) EOL:

cycle	latest	release	eol
3.10	3.10.0	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

Also update some other CI config like using actions/setup-python's own pip caching instead of using actions/cache directly.

TODO

• Hopefully we can release the Deprecate private functions #234 deprecations first
• and also include their removals in this major version bump: Remove deprecated private function aliases #241
• Merge Remove deprecated VERSION, use __version__ instead #253 first
• After release, update https://github.com/conda-forge/humanize-feedstock/blob/master/recipe/meta.yaml

[codecov-commenter]

Codecov Report

Merging #239 (1647180) into master (309e7fd) will decrease coverage by 0.29%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #239      +/-   ##
==========================================
- Coverage   97.65%   97.35%   -0.30%     
==========================================
  Files          10       10              
  Lines         681      681              
==========================================
- Hits          665      663       -2     
- Misses         16       18       +2     

Flag	Coverage Δ
GHA_Ubuntu	96.18% <ø> (-0.30%)	⬇️
GHA_Windows	95.88% <ø> (-0.30%)	⬇️
GHA_macOS	96.18% <ø> (-0.30%)	⬇️

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Impacted Files	Coverage Δ
tests/test_humanize.py	88.88% <0.00%> (-11.12%)	⬇️
src/humanize/__init__.py	94.73% <0.00%> (-5.27%)	⬇️

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