Drop Python 3.9 support

[cuu508]

What do these changes do?

Drop Python 3.9 references from README, GHA ci-cd workflow, tox.ini, setup.cfg

Are there changes in behavior for the user?

Yes – no more Python 3.9 support.

Related issue number

Checklist

• I think the code is well written
• Unit tests for the changes exist
• tox testenvs have been executed in the following environments:
  • Linux (Ubuntu 18.04, Ubuntu 20.04, Arch): {py36,py37,py38,py39}-{nocov,cov,diffcov}, qa, docs
  • Windows (7, 10): {py36,py37,py38,py39}-{nocov,cov,diffcov}
  • WSL 1.0 (Ubuntu 18.04): {py36,py37,py38,py39}-{nocov,cov,diffcov}, pypy3-{nocov,cov}, qa, docs
  • FreeBSD (12.2, 12.1, 11.4): {py36,pypy3}-{nocov,cov,diffcov}, qa
  • Cygwin: py36-{nocov,cov,diffcov}, qa, docs
• Documentation reflects the changes
• Add a news fragment into the NEWS.rst file
  • Add under the "aiosmtpd-next" section, creating one if necessary
    • You may create subsections to group the changes, if you like
  • Use full sentences with correct case and punctuation
  • Refer to relevant Issue if applicable

[greptile-apps]

Confidence Score: 4/5

The PR appears safe to merge, with a non-blocking documentation follow-up needed to align the pull-request checklists with the new support matrix.

The packaging floor and automated matrices consistently drop Python 3.9, but the unchanged contributor templates continue directing authors to validate only obsolete Python environments.

Files Needing Attention: tox.ini and the two pull-request templates

_{Reviews (1): Last reviewed commit: "Drop Python 3.9 support" | Re-trigger Greptile}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.79%. Comparing base (ea98162) to head (74ee081).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #629      +/-   ##
==========================================
- Coverage   97.81%   97.79%   -0.02%     
==========================================
  Files          23       23              
  Lines        5715     5715              
  Branches      352      352              
==========================================
- Hits         5590     5589       -1     
- Misses         75       77       +2     
+ Partials       50       49       -1     

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[webknjaz]

Let's merge this in October or after making a release around that time.