Remove python 3.9

[gadomski]

Description:

Numpy ended support Apr 05, 2024. We've done these in MINOR releases in the past, so if https://github.com/stac-utils/pystac/milestone/37 requires a MAJOR we'll want to get a MINOR out w/ this change first, I reckon.

PR Checklist:

• pre-commit hooks pass locally
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.28%. Comparing base (65ea9a9) to head (aa81156).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1384      +/-   ##
==========================================
+ Coverage   91.07%   91.28%   +0.21%     
==========================================
  Files          51       51              
  Lines        7070     7220     +150     
  Branches     1012     1018       +6     
==========================================
+ Hits         6439     6591     +152     
+ Misses        451      450       -1     
+ Partials      180      179       -1     

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[jsignell]

This looks good to me!