allow working without all backends

[thiswillbeyourgithub]

• feat: make susbai work if missing dependencies
• enh: regroup the dependencies per backend as needed
• add ffmpeg-python as a dep
• doc: add python installation instructions

Hi,

Neat project!

I had some trouble installing it and I just needed a single backend so I figured I'd make a PR to make subsai work even if some dependencies are missing.

Well no actually I initially wanted to do this for myself only but after seeing how well organized the configs.py file was I decided to contribute :)

Let me know what you think.

Bye and thanks.

[absadiki]

Hi @thiswillbeyourgithub,

I actually wanted to do this myself for a long time, so I really appreciate you taking the initiative and making it happen.
Everything looks great.

Thanks a lot for the contribution :)