Fixe TypeError

[PetrDlouhy]

I was getting TypeError: __exit__() takes 1 positional argument but 4 were given with following Django code:

          with no_connection():
              o = c.recalculate(10, BillingInfo(tax_number='48136450', country='CZ'))
              self.assertEqual(o.tax, 21)

This MR fixes that problem.