Switch to python 3.9 in dev-deps CI matrix run since dev deps are now beginning to drop Python 3.8 which is EOL soon

[asmacdo]

Fixes #1503

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.63%. Comparing base (d5c4a3f) to head (faff2f6).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1506      +/-   ##
==========================================
+ Coverage   88.56%   88.63%   +0.07%     
==========================================
  Files          77       77              
  Lines       10572    10572              
==========================================
+ Hits         9363     9371       +8     
+ Misses       1209     1201       -8     

Flag	Coverage Δ
unittests	88.63% <ø> (+0.07%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

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

[yarikoptic]

added a label for auto , and adjusted PR title so it is descriptive of the change whenever it gets into CHANGELOG. dev-deps is green, let's proceed