Bump default target Python version to 3.9

[ncoghlan]

Python 3.8 reached end-of-life in October 2024.

typing.Annotated was added in Python 3.9, so using annotations will no longer need typing_extensions by default.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (dd92cd0) to head (207e2c2).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #2271   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           38        38           
  Lines         4307      4307           
  Branches       994       994           
=========================================
  Hits          4307      4307           

Flag	Coverage Δ
unittests	99.67% <ø> (ø)

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[ncoghlan]

Not an urgent change, I just got bitten by other projects no longer depending on typing-extensions in Python 3.13, which meant I noticed my own dependency that I had inadvertently picked up by generating code compatible with a Python version I didn't support anyway.

[ncoghlan]

Reverted to draft, as the CI failure indicates there's more to this change than just bumping the default value.

[codspeed-hq]

CodSpeed Performance Report

Merging #2271 will not alter performance

_{Comparing ncoghlan:patch-1 (207e2c2) with main (dd92cd0)}

Summary

✅ 31 untouched benchmarks