CI: Begin testing on Python 3.12

[effigies]

Numpy 1.26b1 is out, allowing tests on Python 3.12.

Not yet ready to test on numpy 2.0, so leaving the nightly build URLs as-is for now.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.02% ⚠️

Comparison is base (5f37398) 92.16% compared to head (a42321f) 92.15%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1247      +/-   ##
==========================================
- Coverage   92.16%   92.15%   -0.02%     
==========================================
  Files          98       98              
  Lines       12371    12374       +3     
  Branches     2542     2542              
==========================================
+ Hits        11402    11403       +1     
- Misses        646      648       +2     
  Partials      323      323              

Files Changed	Coverage Δ
nibabel/openers.py	93.10% <100.00%> (+0.18%)	⬆️

... and 1 file with indirect coverage changes

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[effigies]

Well, looks like there are numeric issues, or possibly isinstance() or similar checks are now producing different results. Debugging locally seems tricky, as Python 3.12rc1 won't build with pyenv on Ubuntu 22.04. Will try a Docker image when I get a chance.

[effigies]

Not a numeric issue, but gzip: python/cpython#108111