This makes pythonPackages.sqlalchemy the most up to date revision (it
was called sqlalchemy_1_0 before), and maintains the various “legacy”
versions available as pythonPackages.sqlalchemyX for X in {7,8,9}.
All derivations that required `sqlalchemy_1_0` now require `sqlalchemy`
while those that required `sqlalchemy` now require `sqlalchemy7`.
The derivations are not changed, only the attribute names they are
bound to.
