Drop python 3.6

[CaselIT]

I think we need to remove support for python 3.6, since the current main uses from __future__ import annotation that raises SyntaxError: future feature annotations is not defined on py 3.6

@zzzeek this probably requires 1.8?

[zzzeek]

yes let's do a bump, since have a lot of installation types of things in the queue

[sqla-tester]

CaselIT has proposed a fix for this issue in the main branch:

Alembic 1.8 now supports Python 3.7 and above https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/3819