Drop support for EOL Python 3.6

[hugovk]

Python 3.6 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

Also add python_requires metadata to help pip and upgrade Python syntax with https://github.com/asottile/pyupgrade/

[codecov[bot]]

Codecov Report

Merging #378 (a575d63) into master (f316b6c) will decrease coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #378      +/-   ##
==========================================
- Coverage   85.30%   85.28%   -0.02%     
==========================================
  Files          16       16              
  Lines        1313     1312       -1     
  Branches      137      199      +62     
==========================================
- Hits         1120     1119       -1     
  Misses        146      146              
  Partials       47       47              

Impacted Files	Coverage Δ
...ewsletter/management/commands/submit_newsletter.py	60.00% <ø> (-1.91%)	:arrow_down:
newsletter/settings.py	92.45% <ø> (ø)
newsletter/addressimport/parsers.py	69.93% <100.00%> (ø)
newsletter/admin.py	79.04% <100.00%> (ø)
newsletter/models.py	91.66% <100.00%> (ø)
newsletter/views.py	96.98% <100.00%> (ø)

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