Add greenlet module in the dependency

[sdglitched]

Took the liberty to add the greenlet module in the dependencies to fix issue #106 – now the database initializes smoothly!

[abompard]

We don't use greenlet ourselves, so I don't think it's the right fix to add it to our dependencies.
I think we should depend on sqlalchemy[asyncio], the extra is going to bring in greenlet and it's what we're actually using.