See psf/black#3302 for details.

Fixes: #37
Signed-off-by: Stanislav Levin <slev@altlinux.org>

https://docs.python.org/3/library/unittest.mock.html#unittest.mock.Mock
> side_effect: A function to be called whenever the Mock is called. See
  the side_effect attribute. Useful for raising exceptions or dynamically
  changing return values. The function is called with the same arguments
  as the mock, and unless it returns DEFAULT, the return value of this
  function is used as the return value.
  Alternatively side_effect can be an exception class or instance. In
  this case the exception will be raised when the mock is called.

Fixes: #39
Signed-off-by: Stanislav Levin <slev@altlinux.org>