Drop support for python 3.6 and 3.7

[parthea]

Created a bug to track dropping support for python 3.6 and 3.7.

Once we drop python 3.6 and 3.7, we should update the ubuntu version to the latest in our github actions as suggested in #385

proto-plus-python/.github/workflows/tests.yml

Line 42 in 9d52442

runs-on: ubuntu-20.04