Make gc check resilient to broken weakrefs

In some python deployments (I believe macOS is one), weakref objects aren't guaranteed to exist across gc collections. The proposed check pattern is also used in tensorflow [0] which ran into similar issues a few years ago.

[0]: https://github.com/tensorflow/tensorflow/blob/master/tensorflow/python/framework/errors_test.py#L33

Author: msuozzo

tests/unit/test_dbapi_connection.py

@@ -219,8 +219,14 @@ def test_does_not_keep_cursor_instances_alive(self):
         # Connections should not hold strong references to the Cursor instances
         # they created, unnecessarily keeping them alive.
         gc.collect()
-        cursors = [obj for obj in gc.get_objects() if isinstance(obj, Cursor)]
-        self.assertEqual(len(cursors), 2)
+        cursor_count = 0
+        for obj in gc.get_objects():
+          try:
+            if isinstance(obj, Cursor):
+              cursor_count += 1
+          except ReferenceError:
+            pass
+        self.assertEqual(cursor_count, 2)
 
     def test_commit(self):
         connection = self._make_one(client=self._mock_client())