Requirement management via requirements.txt or pipenv

[lemonsaurus]

This repo should use either a simple requirements.txt file, or a system like pipenv to manage its requirements. This makes it easier for contributors to set up a test environment, and takes the guesswork out of which versions you're currently using. For example, it's currently hard to determine if you're using the discord.py async or rewrite branch (which are very different), because they both use import discord.