Adapt to functools.partial becoming a method descriptor in Python 3.14

[musicinmybrain]

From https://docs.python.org/dev/whatsnew/3.14.html#changes-in-the-python-api:

functools.partial is now a method descriptor. Wrap it in staticmethod() if you want to preserve the old behavior. (Contributed by Serhiy Storchaka and Dominykas Grigonis in gh-121027.)

This was reported downstream in Fedora as https://bugzilla.redhat.com/show_bug.cgi?id=2322407.

It’s hard to test this change in a virtualenv since there are some dependencies like numpy that are nontrivial to compile from source and haven’t published binary wheels for Python 3.14. However, I tested this PR as a patch for nibabel 5.3.2 in Fedora’s early Python 3.14 testing and got a successful build with no regressions using Python 3.14.0a2.

[musicinmybrain]

Adjusted the commit message to note that this fixes #1390.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.37%. Comparing base (d9c479a) to head (bc216da).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1391   +/-   ##
=======================================
  Coverage   95.37%   95.37%           
=======================================
  Files         207      207           
  Lines       29672    29672           
  Branches     3018     3018           
=======================================
  Hits        28300    28300           
  Misses        934      934           
  Partials      438      438           

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