Everything that I learned about it through a search was either incorrect or incomplete. So how am I:
This works for me:
1) Access the session, just like you did session.create_all, do session.drop_all.
2) Delete the migration files generated by alembic.
3) Run session.create_all and the initial migration.
I don't know how to tinker with alembic, but for the database you can just go into the SQL console and use DROP DATABASE foo.
DROP DATABASE foo
, ? , postgres .