Declare support for Python 3.8+ only

[bitprophet]

Rest of my kit is becoming 3.6+ only for now (tho yes I intend to speed up the EOL cadence somewhat); do you anticipate any outright breakage on 3.6/3.7 @AA-Turner ?

[bitprophet]

Bah, I managed to cock up the branches when pushing some old changes of mine, so I'll be re-merging this and the others. I may keep 3.6 listed for now, just to be a jerk.