RuntimeError: generator raised StopIteration in python 3.7 in unit test

Hi,

When building Murano Rocky RC1 in Debian Sid, the below unit test fails. This is specific to Python 3.7, which is available in both Ubuntu and Debian.

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FAIL: murano.tests.unit.dsl.test_helpers.TestDSLHelpers.test_walk_gc_with_towards_true
murano.tests.unit.dsl.test_helpers.TestDSLHelpers.test_walk_gc_with_towards_true
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_StringException: Traceback (most recent call last):
  File "/<<PKGBUILDDIR>>/murano/dsl/helpers.py", line 741, in walk_gc
    [(t, trace + [item]) for t in gc.get_referrers(item)]
  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 "/usr/lib/python3/dist-packages/mock/mock.py", line 1305, in patched
    return func(*args, **keywargs)
  File "/<<PKGBUILDDIR>>/murano/tests/unit/dsl/test_helpers.py", line 313, in test_walk_gc_with_towards_true
    for obj in helpers.walk_gc(first_obj, True, handler):
RuntimeError: generator raised StopIteration