Set up python packaging for pypi distribution (#424)

Old GitHub CI using Ubuntu and MacOS explicitly and building wheels have
been removed in favour of the more portable pypa specified builds. These
wheels should work just as well across a wider range of distributions.

pybind11:CMakeLists.txt requires Development.Module instead of
Development.* to avoid Embed from getting in the way of manylinux
builds.

manylinux_x86_64 builds are added for cp3.6 - 3.10. The linux build
uses an old image via docker.  Since the docker images are able to use
shared ccache folder, builds quite fast on warm starts.

ccache usage in setup.py is now triggered by an environment variable.
This allows for builds not to fail if ccache not present.

On tag pushes corresponding to versions, CI is configured to deliver
built wheels to PyPI, reading from repository secrets.

Improves setup.py including documentation and some formatting, and
additional links to source.

Fixes: #315