Remove scikit-learn extension as submodule, publish independently instead

[PGijsbers]

Metadata

• Reference Issue: [DRAFT] [Megathread] Removing the sklearn extensions from the openml-python code base #1408, a number of transferred issues
• New Tests Added: No
• Documentation Updated: No, I figure @LennartPurucker will happily take care of that 😊
• Change Log Entry: The scikit-learn submodule is now available as a separate package. Install openml-sklearn in order to use run_model_on_task and other extension functions with scikit-learn objects.

Details

The scikit-learn extension repository is here: https://github.com/openml/openml-sklearn
It is now used as a dependency for tests only, which means that scikit-learn will no longer be a dependency of openml-python itself.
Next steps are to remove dependency on the scikit-learn extension as much as possible for tests.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 42.85714% with 8 lines in your changes missing coverage. Please review.

Project coverage is 52.75%. Comparing base (6103874) to head (0a34613).

Files with missing lines	Patch %	Lines
openml/extensions/functions.py	11.11%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1424      +/-   ##
===========================================
- Coverage    53.71%   52.75%   -0.97%     
===========================================
  Files           38       36       -2     
  Lines         5229     4343     -886     
===========================================
- Hits          2809     2291     -518     
+ Misses        2420     2052     -368     

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[LennartPurucker]

LGTM, one small question