[3.6] bpo-31418: Fix an assertion failure in PyErr_WriteUnraisable() in case of an exception with a bad __module__ attribute. (GH-3539) (#3556)

(cherry picked from commit f6e61df)

Author: miss-islington

Misc/NEWS.d/next/Core and Builtins/2017-09-13-13-03-52.bpo-31418.rS-FlC.rst

@@ -0,0 +1,2 @@
+Fix an assertion failure in `PyErr_WriteUnraisable()` in case of an
+exception with a bad ``__module__`` attribute. Patch by Oren Milman.

Python/errors.c

@@ -978,7 +978,7 @@ PyErr_WriteUnraisable(PyObject *obj)
     }
 
     moduleName = _PyObject_GetAttrId(t, &PyId___module__);
-    if (moduleName == NULL) {
+    if (moduleName == NULL || !PyUnicode_Check(moduleName)) {
         PyErr_Clear();
         if (PyFile_WriteString("<unknown>", f) < 0)
             goto done;