Comment 1 for bug 1221467

This new argument name "type_" is involved from alembic 0.5.0 version:

root@ubuntu:/usr/local/lib/python2.7/dist-packages/alembic# grep drop_constraint *.py
operations.py: self.impl.drop_constraint(constraint)
operations.py: def drop_constraint(self, name, table_name, type_=None, schema=None):
operations.py: self.impl.drop_constraint(const)
root@ubuntu:/usr/local/lib/python2.7/dist-packages/alembic# cat __init__.py |grep version
__version__ = '0.5.0'
root@ubuntu:/usr/local/lib/python2.7/dist-packages/alembic#

So, the problem is caused by a lower version of alembic, we can try to install alembic >= 0.5.0 to fix this issue.