Replace setup.py with pyproject.toml

[rgaiacs]

Partially related with https://github.com/jupyterhub/repo2docker/issues/1427

The project uses setup.cfg and setup.py for installation.

pyproject.toml was defined almost 10 years ago in PEP 518.

Should we adopt pyproject.toml?

[manics]

Sounds fine to me!

[minrk]

please go ahead!

[lixiname]

I submitted a PR to migrate from setup.py to pyproject.toml, but it hasn't been merged yet.

[rgaiacs]

Dear @lixiname, Thanks for the pull request.