RF: Purge pkg_resources, add data loader

[effigies]

Enable nireports to be installed and tested on Python 3.12, which no longer bundles setuptools in its virtual environments.

Apparently we're advertising 3.12 support without testing it... Adding that in.

Updating CircleCI, since we weren't installing the package there. I don't think we need the big Docker image.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (19e1536) 57.36% compared to head (87dc5d7) 57.36%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #85   +/-   ##
=======================================
  Coverage   57.36%   57.36%           
=======================================
  Files          22       22           
  Lines        2024     2024           
  Branches      394      359   -35     
=======================================
  Hits         1161     1161           
  Misses        782      782           
  Partials       81       81           

Flag	Coverage Δ
unittests	57.14% <100.00%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[effigies]

@celprov @oesteban @mgxd I'm comfortable merging this tomorrow without review, but feel free to have a look.

[mgxd]

Looks good - I am once again advocating for a standalone nipreps Loader package 😄