remove end-of-life python 3.9 from support matrix

[glyph]

fixes #940

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.91%. Comparing base (cdcb0cd) to head (9573758).

Additional details and impacted files

@@             Coverage Diff             @@
##             trunk     #941      +/-   ##
===========================================
- Coverage   100.00%   99.91%   -0.09%     
===========================================
  Files           63       63              
  Lines         4857     4830      -27     
  Branches       201      201              
===========================================
- Hits          4857     4826      -31     
- Misses           0        3       +3     
- Partials         0        1       +1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[adiroiban]

All good. thanks!

[adiroiban]

This is ok.
One minor optimization, that we have in twisted/twisted.

You can define a global env variable for "default python version" and the use it everywhere.

In this way, future updates will require fewere changes

[glyph]

Thanks for calling this out; I think I'll fix it in a follow-up and land this as-is. Hopefully it will be less error-prone next time :)