chore: Drop Python 3.9 support

[effigies]

I've set up tox to handle Python 3.14, but we're waiting on scikit-image to publish wheels.

Possibly it will build in CI and we can test now, but I'm not going to complicate this PR. Will open a separate one that can be merged when it's convenient.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.20%. Comparing base (fa0032f) to head (36e147c).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
niworkflows/func/tests/test_util.py	0.00%	1 Missing ⚠️
niworkflows/interfaces/images.py	0.00%	1 Missing ⚠️
niworkflows/interfaces/itk.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #988      +/-   ##
==========================================
- Coverage   71.54%   71.20%   -0.35%     
==========================================
  Files          89       89              
  Lines        8653     8578      -75     
  Branches     1077      931     -146     
==========================================
- Hits         6191     6108      -83     
- Misses       2230     2238       +8     
  Partials      232      232              

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