Add a requirements.txt file

[TheLostLambda]

Hello!

Largely inspired by issue #11, and otherwise inspired by the general pain that sorting Python dependencies causes me, I think it might be helpful to check a requirements.txt file into the repo! https://www.freecodecamp.org/news/python-requirementstxt-explained/

That way new users could install all of the needed dependencies (the correct versions every time — avoiding breakage like that in #11) with a single pip install -r requirements.txt or similar.

Thanks again for the lovely project!
Brooks

[TheLostLambda]

I realised that it wouldn't take much time at all to make a PR, so I whipped one up that fixes this issue and #11!

Hopefully it's helpful!