Bump Up Minimum Python Supported Version to 3.8

[gfrances]

Python 3.8 is already more than a year old, ships by default in the latest Ubuntu LTS and, more relevant to us, has a number of improvements that would allow us to clean up the code a little bit and

• Make the use of mypy and similar static type-checking tools more systematic and simpler.
• Get rid of the multipledispatch dependency, and replace it by functools.simpledispatch.

This issue is to keep track of moving towards Python 3.8.