Switch from setup.cfg/setup.py to pyproject.toml

[leplatrem]

Repo modernization!

• Merge Get rid of tox #3357
• Switch from setup.cfg/setup.py to pyproject.toml
• Use requirements.in to build the constraints file. This is messy, and would probably lead us to a wrapper like Poetry, in order to have a proper lock file. Because pip does not support hashes in editable mode, and pip-compile does not pick extra dependencies from pyproject.toml, they therefore have to be duplicated in a requirements.in file.
• Update release docs. One-click release! But I left the 3 steps that consist in checking that everything is ready and that everything was published correctly
• Check that entry points still work
• Fix Docker container build (see Run functional tests on freshly built container #3367)

[leplatrem]

I noticed the docker image build is broken in this PR, I'll open a separate PR to run the functional tests against a freshly docker image

[grahamalama]

Curious, what is the difference between these dependencies and those listed in pyproject.toml?

[leplatrem]

This requirements.in is just the "source" of our constraints file.

And the dependencies in pyproject.toml are the final package dependencies (when installed from pip for example).

I will open an issue about that, because I'm not happy about a single requirements.txt as a constraints file, for both our main and our extra dependencies.