rf(py314): Replace deprecated pkgutil.find_loader

[effigies]

This PR removes pkgutil.find_loader() and replaces it with importlib.util.find_spec(). find_loader was deprecated in Python 3.12 and will be removed in 3.14. find_spec has been present since Python 3.4.

Both functions return None if the module loader cannot be found. For its use in this project, this is sufficient and no translation of the return value is needed.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.06%. Comparing base (5d7a6d3) to head (bf38d9e).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
fmriprep/cli/parser.py	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3384   +/-   ##
=======================================
  Coverage   72.06%   72.06%           
=======================================
  Files          57       57           
  Lines        4231     4231           
  Branches      456      455    -1     
=======================================
  Hits         3049     3049           
  Misses       1067     1067           
  Partials      115      115           

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