Support for PEP 517/518 (pyproject.toml build-system)

[starhel]

PEP 517/518 (as well as their successors) are widely adapted to the python ecosystem and have become standard in how Python distribution builds are declared and executed¹. Because of the last pip deprecation (https://github.com/pypa/pip/issues/8559) I'm opening this issue.

I've made some tests and luigi could be easily refactored to setup.cfg + pyproject.toml². Unfortunately, there are some issues to solve:

• Moving all metadata to setup.cfg requires using importlib.metadata (or its backport) to generate luigi.__meta__. Backport is available from python 3.6, so I was forced to drop python 3.5 support (EOL Sep 2020).
• I've never uploaded docs to readthedocs.io, so I don't know if the READTHEDOCS environment param is still used and necessary (as tox -e dev does not use it to locally build docs).

If you are ok with dropping support for pythons older than 3.6 and removing READTHEDOCS env param, I can prepare PR with all necessary changes. When all changes will be done, deploying new versions should also enforce building wheel distribution (it's a single step using build). If you want I could also prepare a GitHub Action workflow to publish packages to PyPI and GH releases based on pushing tags to the repository.

1. More about avoiding setup.py: https://setuptools.pypa.io/en/latest/userguide/quickstart.html#setuppy-discouraged
2. I'm aware that setuptools supports now also pyproject.toml configuration, but many features are still in the beta stage.