Drop support for Python 3.6, 3.7, and 3.8 (since past EOL)

[rnag]

In next iteration (probably minor release) I want to drop support for Python 3.6, 3.7, and 3.8. Check out the Python End of Life Cycle here.

This will also have other benefits, such as:

• fix pyup errors - currently pyup shows as "insecure" since we're forced into using old or outdated pkg versions since most don't support Python 3.6
• clean up most if not all TODOs scattered in code in library, since most of those are python version- specific anyway.