Refactor for future developers

[willingc]

To make it easier for future developers on reactivepy, let's refactor a few things:

• Be consistent with f-strings since this supports 3.6+
• Clean up import statements (order by stdlib, 3rd party packages, reactivepy imports)
• Add metadata for pypi to setup.py (pypa has an example repo and jupyterhub is a good model)
• Go through and add docstrings that would be helpful for a future developer (they are also very handy for autodocumenting things) - Your 6 month from now self will thank you ;-)

[declanvk]

f-string stuff fixed by #24 #23 #22

[richagadgil]

libraries fixed by #21

[ellisonbg]

Thanks @willingc !