build: drop python 3.8 add py3.13

[tlambert03]

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[codecov]

Codecov Report

Attention: Patch coverage is 97.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.97%. Comparing base (ca6b595) to head (050d3df).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/magicgui/_type_resolution.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #671      +/-   ##
==========================================
- Coverage   89.16%   88.97%   -0.20%     
==========================================
  Files          39       39              
  Lines        4746     4743       -3     
==========================================
- Hits         4232     4220      -12     
- Misses        514      523       +9     

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[tlambert03]

note that I'm removing one test here:

    assert resolve_single_type(Optional['List["numpy.ndarray"]']) == Optional[List[np.ndarray]]

Optional['List[numpy.ndarray]'] will work, but you can't add an arbitrarily nested number of strings