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

[leplatrem]

Repo modernization!

• [x] Merge #3357
• [x] Switch from setup.cfg/setup.py to pyproject.toml
• [x] 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.
• [x] 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
• [x] Check that entry points still work
• [x] Fix Docker container build (see #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