Pyproject and setuptools

[jmahlik]

Code of Conduct

Description

Adding the basics for pyproject support and fixes installation from sdist.

Related issues or pull requests

Related https://github.com/rasbt/mlxtend/issues/1062

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)
• [x] 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

[rasbt]

Thanks a lot for the PR. At first glance, the version handling looks fine to me. At some point, I think it could make sense to move it back to a hardcoded version though 😅.

[jmahlik]

It looks like conda is unhappy updating the environment in ci.

I'll see what can be done about the version in another PR. Might be able to use the "attr" feature of pyproject.toml once all the metadata is over there.

[rasbt]

Arg, yes, dependency management is the most frustrating part about CI setups. It seems to be a new compatibility issue (probably unrelated with the changes in your PR) with the pinned NumPy and Pandas versions. It's weird as it worked fine before.