Consider dropping support for PyPy3 in nimi-python

[marcoskirsch]

Supporting PyPy officially is not trivial for us.

• We aren't doing it correctly now nimi-bot log says "SKIPPED: InterpreterNotFound: pypy3" #1260
• It doesn't support 64-bit Windows. This forces us to allow skipping environments in tox for system testing which means real problems could go on undetected (like a bad VM setup)
• They are behind in Python language support, at 3.6 (CPython is at 3.8).
• There is no pre-compiled numpy/scipy
  • So we need to either introduce a mechanism to skip those tests during system testing
  • Or we'd need to compile and install at execution time

Based on analysis of PyPI downloads I did back in October, out of 38,000 cumulative downloads of all NI Python modules there was a single (1!) download made from PyPy as the Python implementation:

2019-05-18 00:09:43 nidaqmx version 0.5.7 on Windows 7

So I propose we remove official support and stop spending efforts properly testing it and instead work on more valuable things.

Customer feedback will be taken very seriously here.

[marcoskirsch]

Update: We have a approval to do this.