Python 3 compatibility

[bstabler]

as suggested by @e-lo

[bstabler]

Python 2.7 will be retired Jan 1, 2020 - see https://pythonclock.org/

[guyrousseau]

Somewhat related to this, ArcGIS 10.5 requires Python 2.7.13. It seems as if ESRI is running the clock out on Desktop, which 10.6 looks like the last version, which will prompt a switch over to Pro, which supports Python 3.x.

[bstabler]

We decided to support both 2.7 and 3. We will gradually make the necessary updates during the course of the Phase 4 work.