Drop Python 2 support #24

I would suggest dropping Python 2 support in the next release of the Python-Extension. Python 2.7 is now end-of-life for almost 3 years, and practically all scientific packages has dropped support for Python 2. It will make maintenance, packaging and testing easier.

The next minimum supported (and tested) Python version could be Python 3.6 or 3.7. NEP 29, the NumPy version support policy, even suggests bumping directly to 3.8+, so that could also be an option. I think it's not much effort to support 3.6 and 3.7 for some time also however.