bpo-34042: Fix dict.copy() to maintain correct total refcount (GH-8119)

(cherry picked from commit 0b75228)

Co-authored-by: Yury Selivanov <yury@magic.io>

Author: miss-islington

Misc/NEWS.d/next/Core and Builtins/2018-07-05-15-51-29.bpo-34042.Gr9XUH.rst

@@ -0,0 +1,2 @@
+Fix dict.copy() to maintain correct total refcount (as reported by
+sys.gettotalrefcount()).

Objects/dictobject.c

@@ -656,6 +656,13 @@ clone_combined_dict(PyDictObject *orig)
         /* Maintain tracking. */
         _PyObject_GC_TRACK(new);
     }
+
+    /* Since we copied the keys table we now have an extra reference
+       in the system.  Manually call _Py_INC_REFTOTAL to signal that
+       we have it now; calling DK_INCREF would be an error as
+       keys->dk_refcnt is already set to 1 (after memcpy). */
+    _Py_INC_REFTOTAL;
+
     return (PyObject *)new;
 }