Restore Python 3.6+ compatibility

[fernandobrito]

Resolves #52

• Reverts using Literal so the project runs on Python 3.6+
• Uses Python 3.6.x on the GitHub Actions to make sure the project runs on Python 3.6+

[fernandobrito]

I need some help with this one. I can't build the project on Python 3.6.

It seems the setuptools bundled with this version is too old for setuptools_scm, and you can't add setuptools as a setup_requires. One of the suggestions from the error message is to explicitly install the setup tools (like I am doing in this PR in the GitHub Action to test it out), but what are your thoughts on that?

[fernandobrito]

Another place where I need feedback: the package version during runtime magic that you use from https://pypi.org/project/setuptools-scm/#retrieving-package-version-at-runtime requires importlib.metadata which is only available on Python 3.8+. The page points to a library for backward compatibility. Do we want to use it?

[gouline]

I converted this to draft for you, click "Ready for review" when it's no longer WIP.

[fernandobrito]

Thanks! Ready to be reviewed now.

• For the use of Literal, I've replaced it with an Enum
• For the issue with setuptools_scm from my screenshot above, I took the suggestion from the error message and installed an appropriate version of setuptools on GitHub Actions
• For the issue onimportlib not being available before Python 3.8, I wrapped it on a try block, which worked well locally.

But I'm more than open to suggestions on different approaches to solve those 3 issues.

Also, is it fine if I "Squash and merge" once it's time to merge? The commit history in the branch is a bit messy 😄

[gouline]

Left a couple of initial comments.

@z3z1ma should also review, since most of it is his code.

[z3z1ma]

Everything looks accounted for on the Enum update throughout the package. Great job & thanks.

[gouline]

LGTM, thanks for your efforts 🚀