Update python.py

updated dictionary itteration to create a list for generation, so that tests can be added in the generator functions under python3. This works fine as-is in python2 because python 2 already creates a list, whereas python3 returns an itterator. Forcing a list format for the return fixes python3 to work the same way as python2

_pytest/python.py

@@ -451,7 +451,7 @@ class PyCollector(PyobjMixin, pytest.Collector):
         seen = {}
         l = []
         for dic in dicts:
-            for name, obj in dic.items():
+            for name, obj in list(dic.items()):
                 if name in seen:
                     continue
                 seen[name] = True