Update to python37 only

[cthoyt]

Switch to only Python 3.7 and update all references to 3.6 (Pipfile, docs, readme, etc.)

What 37 only features can we stick in the setup so nothing can happen? Like using f-strings for 36 :)

[scolby33]

from __future__ import annotations

class Whatever:
    @classmethod
    def make_whatever(cls) -> Whatever:
        pass

That'll do it. https://docs.python.org/3/whatsnew/3.7.html#pep-563-postponed-evaluation-of-annotations

P.S. I hate that they named that future just plain "annotations"

[cthoyt]

there you go

[scolby33]

Need to update docs and badges in the readme and the docs.