Remove Python2 support

[AlexanderWells-diamond]

While PythonSoftIOC can support Python2, the code in __main__.py does not work in Python2.

As per discussion with @Araneidae and @thomascobb it's probably not worth fixing it, instead this issue will be to remove Python2 support entirely.

This will involve both a lot of code cleanup (there's a lot of Python version checks littered throughout the code) as well as reviewing any documentation about version support.