MNT: Move nisext imports from distutils to setuptools

[penguinpee]

• TEST: Only use stable argsorts in PARREC tests
• ENH: Warn on invalid MINC2 spacing declarations, treat as missing
• Fix tests for Python 3.12

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (134d469) 92.17% compared to head (65379bb) 92.17%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## maint/5.1.x #1241 +/- ## ============================================ Coverage 92.17% 92.17% ============================================ Files 98 98 Lines 12376 12376 Branches 2544 2544 ============================================ Hits 11407 11407 Misses 646 646 Partials 323 323 ```

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[effigies]

I'm okay with this, assuming you wouldn't prefer just to remove nisext from your package altogether.