Sort out duplicate `setup.py` instances

[daviddrysdale]

#7 added a top-level setup.py file to help pip cope with the code being in a subdirectory.

Need to check whether this is still needed. If it is still required, then some mechanism to keep the two files in sync is needed.

[AA-Turner]

Per https://pip.pypa.io/en/stable/topics/vcs-support/#url-fragments, sub-dir installs now seem to be viable

A