Investigate using the Python limited API

[RobinD42]

From Phil:

I don't know if you are aware of the new support for the Python limited API. When I (recently) started to provide wheels on PyPI I soon realised that this was going to be a huge maintenance effort and binary compatibility was going to be a significant issue for me.

I've now got it to the stage that when a new major release of Python is made I only have to update the SIP wheels. I know you use a local copy of the sip module, but if you used the PyPI wheels you would get (eg) Python v3.7 support without doing anything.

[RobinD42]

See: https://www.python.org/dev/peps/pep-0384/