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]

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 ⚠️

Additional details and impacted files

@@            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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