Add a `pyproject.toml` about mlxtend

[jmahlik]

I also noticed the package is importing itself to find the version. Which makes installing from an sdist not possible (the package isn't yet installed when installing itself).

The warning at the very bottom: https://packaging.python.org/en/latest/guides/single-sourcing-package-version/

I'll adjust that in the pr as well.

[rasbt]

Thanks a lot for opening this issue. Actually, moving to .toml for my packages has been something on my list for quite some time, but I never had the time to get to it. If you could open a PR that would be really appreciated!

[jmahlik]

Thanks a lot for opening this issue. Actually, moving to .toml for my packages has been something on my list for quite some time, but I never had the time to get to it. If you could open a PR that would be really appreciated!

Awesome! I opened a PR with the basics of specifying a build system and got the install working from an sdist for now.

I'd be willing to work on migrating the configs from the setup.py over to the pyproject.toml. Think it might require some work/discussion about how to handle the version. Right now it's dynamic from the module. I can start on another PR to migrate it fully.

[rasbt]

@jmahlik I forgot to mention that I made a new PyPI release a few days ago. Thanks again for the PR restructuring this package to finally use .toml!!