Fixup/rework python tag support

This migrates our packaging closer to the newly upstream intended python-abi-tag packages, without using pybuild ids. Upstream has also dropped creating either packages, or symlinks to the "default" builds. Thus the new world order is to link against -lboost_python27 -lboost_numpy36 -lboost_mpi_python37 and similar. Our old compat symlinks should be dropped, once everything starts iterating and building for all supported python releases. At the same time, i've cleaned up the build process a bit, to use separate stage dirs. Just in case really to prevent any accidental clobbering. Also I've fixed up the python extension module name to follow latest PEP, and install directly into python3/dist-packages.

This is ready to upload imho.

Reviewed everything else, and it looks good. Docs are really nice, and the copyright file is pretty.