Pybuild plugin for flit

This merge request adds a new pybuild plugin for packages that use flit. While there are not many packages that could make use of this right now (there are two: pep517 and jeepney), moving towards being able to build Debian packages based on pyproject.toml is a good step forward for the future.

Other than additions to the pybuild man page there are no changes other than addition of the new plugin.

Testing: I converted pep517, jeepney, and flit to use the plugin and other than having a dist-info directory rather than a since distutils style file, the packages are the same.

Flit is not a candidate for using the plugin in the archive because of a potential bootstrapping problem (currently flit uses flit from the uploaded package to build itself, so no external bootstrapping is required), testing with it enabled me to test that installing scripts in /usr/bin works correctly.

Jeepney uses the pybuild testing features, so I was able to verify that works with a flit package.

The flit plugin is only targeted for python3, but I did build a package that still built for python2 to make sure there wasn't any regression. I also ran the autopkgtest and it passed.

Please let me know if you need anything else.

Scott K