almir: pin the sqlalchemy version used by zope.sqlalchemy to 8

Otherwise, sqlalchemy 9 is used which triggers iElectric/almir#61
pinning it to 7 caused a build error. This does mean that both versions
are on the PYTHONPATH, but it seems to work.

@iElectric what do you think?

EDIT:
I've also set postgresql:///bacula as the default sqlalchemy_engine_url which should work out of the box.