Add support for packages using flit

[leouieda]

Description of the desired feature:

Some packages may use flit or another build backend instead of setuptools. In these cases, the dependency info is probably in pyproject.toml instead of setup.cfg. To add support for these packages, we'd need to add new "source" (install_toml,extras_toml maybe) and appropriate parsers for those fields.

See https://flit.readthedocs.io/en/latest/pyproject_toml.html#pyproject-project-dependencies

Are you willing to help implement and maintain this feature?

Yes but would love help if anyone is interested. This is low priority since our projects mostly use setuptools.