Make sqlalchemy an optional dependency

[camillehuot]

Hi guys

Would you know distutils enough to make sqlalchemy an optional dependency?

Since we have only custom DataLayers, we don't use SQLAlchemy and it pulls too many dependencies (like greenlet which does not work on Alpine without a recompilation... so the full pip install is failing). It's a mess. Anyway SQLAlchemy is just a default implementation and should not be mandatory to use it.

I you can supply a simple and nice way to remove sqlalchemy from the packages pulled by pip, it would be awesome.

Best regards
Camille