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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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