MAINT cosmetic follow-up switch pixi + GitHub Actions

[glemaitre]

• remove pytest to be a runtime dependency
• remove _min_denpendcy.py and use the pyproject.toml instead and packaging
• remove scikit-learn < 1.2 specific code

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.14286% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.59%. Comparing base (4d4cfd3) to head (ab41949). Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
imblearn/ensemble/_forest.py	75.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1100 +/- ## ========================================== + Coverage 86.39% 87.59% +1.20% ========================================== Files 58 56 -2 Lines 4291 4185 -106 Branches 853 833 -20 ========================================== - Hits 3707 3666 -41 + Misses 472 414 -58 + Partials 112 105 -7 ```

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