Python 3.7: gc.get_referrers(a_local_var) is []

The important thing for these tests is that the internal data structures
in objgraph's functions do not retain extra references, so we want to
know that the number of referrers does not increase.  It's less
important to make specific assertions about the actual number of
referrers, which apparently differs across Python versions.

I'd like somebody to explain to me why Python 3.7 no longer includes the
currently executing stack frame in the list of referrers for a local
variable.  Some kind of an optimization?

tests.py

@@ -258,9 +258,10 @@ def test_no_new_reference_cycles(self):
         # count()
         gc.disable()
         x = type('MyClass', (), {})()
-        self.assertEqual(len(gc.get_referrers(x)), 1)
+        before = len(gc.get_referrers(x))
         objgraph.count('MyClass')
-        self.assertEqual(len(gc.get_referrers(x)), 1)
+        after = len(gc.get_referrers(x))
+        self.assertEqual(before, after)
 
 
 class TypestatsTest(GarbageCollectedMixin, unittest.TestCase):
@@ -276,9 +277,10 @@ def test_no_new_reference_cycles(self):
         # typestats()
         gc.disable()
         x = type('MyClass', (), {})()
-        self.assertEqual(len(gc.get_referrers(x)), 1)
+        before = len(gc.get_referrers(x))
         objgraph.typestats()
-        self.assertEqual(len(gc.get_referrers(x)), 1)
+        after = len(gc.get_referrers(x))
+        self.assertEqual(before, after)
 
 
 class TypestatsFilterArguTest(GarbageCollectedMixin, unittest.TestCase):
@@ -335,11 +337,11 @@ def test_new_garbage(self):
         res = objgraph.by_type('MyClass')
         self.assertEqual(res, [x])
         # referrers we expect:
-        # 1. this stack frame
+        # 1. this stack frame (except on Python 3.7 where it's somehow missing)
         # 2. the `res` list
         # referrers we don't want:
         # the ``objects`` list in the now-dead stack frame of objgraph.by_type
-        self.assertEqual(len(gc.get_referrers(res[0])), 2)
+        self.assertLessEqual(len(gc.get_referrers(res[0])), 2)
 
 
 class StringRepresentationTest(GarbageCollectedMixin,