Support pyproject.toml

[rgaiacs]

https://repo2docker.readthedocs.io/en/latest/config_files.html does not list support to pyproject.toml and, also, I could not find any mention on GitHub.

[minrk]

I think it makes sense to support PEP 517 and/or PEP 660, following the spec here.

I think it would mean:

1. select Python version based on project.requires-python
2. build PEP 517 via pip install . (or editable PEP 660 via pip install -e .)

A tricky bit would be that there are technically some repos with pyproject.toml that do not define a package (e.g. use it just for tool config), which we would have to deal with (e.g. if build-system.requires is undefined, ignore pyproject.toml).

Another hiccup is that [project] is also optional, as the build system can technically totally populate this, though it is mostly deprecated. This does still come up in some older setuptools projects, where setup.py, which means requires-python is not known until after python3 -m build . makes a wheel. We don't have to support that, though, but it would mean we should also ignore pyproject.toml without a [project] table.