Make this repo an installable Python package

> Maybe an idea for the future. What about we just install `programapi` as a Python package? And have a following functionality:
> * `programapi download`
> * `programapi transform`
> By the way do you think that we can avoid saving unnecessary / private fields when downloading?

_Originally posted by @Kislovskiy in https://github.com/EuroPython/programapi/pull/6#discussion_r1621382933_