Remove 3.6, 3.7, 3.8 from project's metadata and related dead code in the code base

[jmao-denver]

This is to reflect that we no longer build distributions for these versions going forward, so no new bug fixes/features will be available for them. That said, if needed, users most likely should be able to port these changes themselves if the bug fixes or new features are not tied to newer versions of Python.

Fixes #213

[jmao-denver]

@rcaudy We want you to be aware of this change since it potentially impacts the scope of the maintenance of the product, and give you a chance to raise any objection to it.

[devinrsmith]

Is this precise enough? I'm assuming we should say we only support 3.x, and not a theoretical 4.x.

[jmao-denver]

I wouldn’t be concerned about Python 4. Not only is it unlikely to arrive anytime soon—if ever—but the Python core developers have made it clear they’ve learned from the painful transition between Python 2 and 3. If a Python 4 does eventually happen, preserving backward compatibility will be a top priority. The evolution from Python 3 onward is designed to be gradual, stable, and minimally disruptive.