Workaround for PyPy test failure

gc_collect needs to be called twice to persuade PyPy to run the
destructors we care about.

Author: bmerry

tests/test_call_policies.py

@@ -100,6 +100,7 @@ class Derived(Parent):
     with capture:
         del p, lst
         pytest.gc_collect()
+        pytest.gc_collect()    # Needed to make PyPy free the child
     assert capture == """
         Releasing parent.
         Releasing child.