chore: Drop support for Python 3.9

[GabDug]

Note

Low Risk
Primarily a support-matrix change (dropping Python 3.9) plus straightforward cleanup; limited functional risk beyond excluding 3.9 users/CI coverage.

Overview
This PR drops Python 3.9 support by bumping requires-python to >=3.10 and updating documented supported versions.

CI/dev tooling is aligned by removing Python 3.9 from .mise.toml and the tox/tox-gh env matrices (including coverage env wiring), and tests are updated to use >=3.10 metadata where they previously forced >=3.9. It also removes the typing_extensions fallback for TypeAlias now that Python 3.10 is the minimum, with pdm.lock updated accordingly.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.13%. Comparing base (b0df414) to head (9a5544c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #70      +/-   ##
==========================================
+ Coverage   96.86%   97.13%   +0.26%     
==========================================
  Files          12       12              
  Lines         701      697       -4     
  Branches       63       62       -1     
==========================================
- Hits          679      677       -2     
+ Misses         19       18       -1     
+ Partials        3        2       -1     

Flag	Coverage Δ
unittests	97.13% <100.00%> (+0.26%)	⬆️

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