mock out methods of base class

How can I mock out the base class to test the rest behaviour of derived class?

# themod/sql.py

class PostgresStore(object):
    def __init__(self, host, port):
        self.host = host
        self.port = port

    def connect(self):
        self._conn = "%s:%s" % (self.host, self.port)
        return self._conn


# themod/repository.py
from .sql import PostgresStore


class Repository(PostgresStore):

    def freak_count(self):
        pass


# tests/test.py
from themod.repository import Repository
from mock import patch 

@patch('themod.repository.PostgresStore', autospec=True)
def patched(thepatch):
    # print(thepatch)
    x = Repository('a', 'b')

    #### how to mock the call to x.connect?
    print(x.connect())

patched()