MAINT: removing python 3.9 support

[bsipocz]

In practice this doesn't really mean much but to clean up the test matrix a little bit.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.62%. Comparing base (6a6c29a) to head (7503d7a).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #229   +/-   ##
=======================================
  Coverage   89.62%   89.62%           
=======================================
  Files          31       31           
  Lines        1253     1253           
=======================================
  Hits         1123     1123           
  Misses        130      130           

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[pllim]

Thanks!