Issue #15365: Make traceback reporting ignore any errors when printing out

the source line.  Such errors can't be reported anyway.  This makes error
reporting work, even if the "io" module can't be loaded.

Python/traceback.c

@@ -344,7 +344,10 @@ tb_displayline(PyObject *f, PyObject *filename, int lineno, PyObject *name)
     Py_DECREF(line);
     if (err != 0)
         return err;
-    return _Py_DisplaySourceLine(f, filename, lineno, 4);
+    /* ignore errors since we can't report them, can we? */
+    if (_Py_DisplaySourceLine(f, filename, lineno, 4))
+        PyErr_Clear();
+    return err;
 }
 
 static int