Python Versions Going Forward

[craig8]

Part of the new modular VOLTTRON is to be able to release things faster but still maintain a stable base for the community. To that end I am proposing that the next version of VOLTTRON released to work with python 3.7 and python 3.8 primarily. Debian buster has python 3.7 as its python3 version by default while ubuntu 20.04 has python 3.8.

Depending upon community responses this can definitely be revised, however this is the starting point that we can go from.

This is a follow on from #2812.