Enable alembic as the main tool for migrations

Enable alembic for all migrations and limit the max number of
sqlalchemy-migration migration. All new migrations must now be in
Alembic!

mediagoblin/db/migration_tools.py

@@ -317,6 +317,8 @@ class RegisterMigration(object):
         assert migration_number > 0, "Migration number must be > 0!"
         assert migration_number not in migration_registry, \
             "Duplicate migration numbers detected!  That's not allowed!"
+        assert migration_number <= 44, ('Alembic should be used for '
+                                        'new migrations')
 
         self.migration_number = migration_number
         self.migration_registry = migration_registry

mediagoblin/gmg_commands/dbupdate.py

@@ -130,7 +130,6 @@ def run_dbupdate(app_config, global_config):
 
     # TODO: Make this happen regardless of python 2 or 3 once ensured
     # to be "safe"!
-    if six.PY3:
     run_alembic_migrations(db, app_config, global_config)