bpo-31418: Fix an assertion failure in PyErr_WriteUnraisable() in case of an…

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

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

+Fix an assertion failure in `PyErr_WriteUnraisable()` in case of an
+exception with a bad ``__module__`` attribute. Patch by Oren Milman.

Python/errors.c

@@ -961,7 +961,7 @@ 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;