Fix test-requirements.txt

mock >= 1.0 changed the behavior for iterable sideeffects. Any
exceptions in an iterable side_effect will now be raised instead of
returned.

With mock < 1.0 tests are failing, because test for the
internal client (TestSimpleClient.test_get_with_retries) use an
iterable sideeffect that raise an URLError on the first request
and pass on the second request.

Change-Id: I568b68c33ff599d7a4f590e2a0eb8caeee4b4d4e