ci: Drop Python 3.9 checks

[tisonkun]

This is required by #2085.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.34%. Comparing base (8112738) to head (47c3bcc).

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk    #2093   +/-   ##
=======================================
  Coverage   83.34%   83.34%           
=======================================
  Files         353      353           
  Lines       81769    81769           
  Branches     8993     8993           
=======================================
  Hits        68146    68146           
  Misses      10797    10797           
  Partials     2826     2826           

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[tisonkun]

We may add a Required task to gather all necessary tasks for convenient like https://github.com/apache/datasketches-rust/blob/583ac180362a18a4f3f57b651bb99d0000e65729/.github/workflows/ci.yml#L113-L129 later.