254 Deprecate Python 3.9

[charles-turner-1]

Closes #254.

Deprecates Python 3.9:

• Removes CI environment for 3.9.
• Updated type hints to use 3.10+ syntax

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.89%. Comparing base (38d4c45) to head (f3cfe41).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #261      +/-   ##
==========================================
- Coverage   97.90%   97.89%   -0.01%     
==========================================
  Files          11       11              
  Lines         810      807       -3     
==========================================
- Hits          793      790       -3     
  Misses         17       17              

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