Most recent scikit-learn results in several failed unit tests

[it176131]

Description

This handles the failing unit tests when scikit-learn is 1.3.1 or higher.

Related issues or pull requests

1090

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Pull Request Checklist

• [x] Added a note about the modification or contribution to the ./docs/sources/CHANGELOG.md file (if applicable)
• [x] Added appropriate unit test functions in the ./mlxtend/*/tests directories (if applicable)
• [ ] Modify documentation in the corresponding Jupyter Notebook under mlxtend/docs/sources/ (if applicable)
• [x] Ran PYTHONPATH='.' pytest ./mlxtend -sv and make sure that all unit tests pass (for small modifications, it might be sufficient to only run the specific test file, e.g., PYTHONPATH='.' pytest ./mlxtend/classifier/tests/test_stacking_cv_classifier.py -sv)
• [x] Checked for style issues by running flake8 ./mlxtend

[it176131]

As noted in the past few commit messages, scikit-learn implemented a StackingClassifier and StackingRegressor in 0.22 with a different API. Because of this, some of the current unit tests fail as they don't expose the necessary attributes.

@rasbt, should the mlxtend versions of the StackingClassifier and StackingRegressor be replaced with the scikit-learn implementations? Or renamed (so to not cause confusion) and modified to pass the tests?

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.36364% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 78.29%. Comparing base (ad06b2d) to head (d61928a). Report is 8 commits behind head on master.

Files	Patch %	Lines
mlxtend/classifier/stacking_cv_classification.py	50.00%	2 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1091 +/- ## ========================================== + Coverage 77.26% 78.29% +1.02% ========================================== Files 200 196 -4 Lines 11297 11140 -157 Branches 1513 1404 -109 ========================================== - Hits 8729 8722 -7 + Misses 2350 2200 -150 Partials 218 218 ```

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[rasbt]

@rasbt, should the mlxtend versions of the StackingClassifier and StackingRegressor be replaced with the scikit-learn implementations? Or renamed (so to not cause confusion) and modified to pass the tests?

I would prefer to keep them as the scikit-learn port is not 1:1 equivalent, and the stacking classifiers here had some advantages (and disadvantages). I can update the unit tests to add them back, no worries!

[it176131]

@rasbt, should the mlxtend versions of the StackingClassifier and StackingRegressor be replaced with the scikit-learn implementations? Or renamed (so to not cause confusion) and modified to pass the tests?

I would prefer to keep them as the scikit-learn port is not 1:1 equivalent, and the stacking classifiers here had some advantages (and disadvantages). I can update the unit tests to add them back, no worries!

Good team work! Once this is merged I'll sync my other branch and hopefully all the tests will pass 🤞