Move sqlalchemy dependency out to extras

[PeterJCLaw]

As far as I can tell, sqlalchemy is only needed for the (essentially optional) Django directory yet is always a dependency of this project. This means that all consumers need to install sqlalchemy even if it's not otherwise used.

While sqlalchemy isn't the largest package in the world, it's not exactly small either.
It would be great if sqlalchemy was an "extra" package so that it could be omitted when not needed.