Drop support for Python 3.6

[ianhelle]

Is your feature request related to a problem? Please describe.
Python 3.6 will reach end of life at the end of 2021. There will be limited support for updates and security fixes.
There are also features of later releases that we would like to use in development.

Describe the solution you'd like
We are proposing to make the minimum supported version of Python 3.8

Describe alternatives you've considered
Python 3.7 should still work but there would be no guarantees