Py3.7 unit test failure

[Corey Bryant]

The following test is failing for Python 3.7:

======================================================================
FAIL: troveclient.tests.test_backups.BackupManagerTest.test_execution_list
troveclient.tests.test_backups.BackupManagerTest.test_execution_list
----------------------------------------------------------------------
_StringException: Traceback (most recent call last):
  File "/<<PKGBUILDDIR>>/troveclient/v1/backups.py", line 240, in mistral_execution_generator
    sort_dirs='desc')
  File "/usr/lib/python3/dist-packages/mock/mock.py", line 1062, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/mock/mock.py", line 1121, in _mock_call
    result = next(effect)
StopIteration

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/<<PKGBUILDDIR>>/troveclient/tests/test_backups.py", line 253, in test_execution_list
    el = self.backups.execution_list("dummy", mistral_client, limit=2)
  File "/<<PKGBUILDDIR>>/troveclient/v1/backups.py", line 259, in execution_list
    return list(execution_list_generator())
  File "/<<PKGBUILDDIR>>/troveclient/v1/backups.py", line 250, in execution_list_generator
    for sexec in mistral_execution_generator():
RuntimeError: generator raised StopIteration