Incomplete "from source" instructions in README

[poopsicles]

I was attempting to make a nix flake for this repo, so I could easily build the emojis reproducibly (you can see it here, though I should probably upstream it here too) and discovered some of the info in the README is slightly wrong:

• A Python 2.x version isn't necessary, Python 3 worked fine for me (I was having trouble getting 2 to work even, because it's EOL, and so I just tried with 3).
• Imagemagick is also a dependency that's needed, as well as which (I assumed which was a shell builtin, but that seems to only be for zsh, which led to me asking this question).

I'd open a PR fixing these, but I'm opening an issue instead because there's probably justification for specifying Python 2.x that I can't see?