CLI should be installable using `pip` for users who only need Python

[drdavella]

Some CLI users may only need Python codemods. In these cases, we should make the CLI usable from a pip installation (without brew). This would mean that pip install pixee would make both the CLI itself and the Python codemodder available without the Java codemodder.

[drdavella]

@nahsra has pointed out that this would break parity between Java and Python installations which would not be desirable.