WindowsError.str should display the windows error code,
not the posix error code; with test.
Fixes #1576174.

Backported from trunk, revision 52485.
diff --git a/Objects/exceptions.c b/Objects/exceptions.c
index c0b813d..0cd819c 100644
--- a/Objects/exceptions.c
+++ b/Objects/exceptions.c
@@ -828,9 +828,9 @@
             return NULL;
         }
 
-        if (self->myerrno) {
-            Py_INCREF(self->myerrno);
-            PyTuple_SET_ITEM(tuple, 0, self->myerrno);
+        if (self->winerror) {
+            Py_INCREF(self->winerror);
+            PyTuple_SET_ITEM(tuple, 0, self->winerror);
         }
         else {
             Py_INCREF(Py_None);
@@ -852,7 +852,7 @@
         Py_DECREF(fmt);
         Py_DECREF(tuple);
     }
-    else if (self->myerrno && self->strerror) {
+    else if (self->winerror && self->strerror) {
         PyObject *fmt;
         PyObject *tuple;
 
@@ -866,9 +866,9 @@
             return NULL;
         }
 
-        if (self->myerrno) {
-            Py_INCREF(self->myerrno);
-            PyTuple_SET_ITEM(tuple, 0, self->myerrno);
+        if (self->winerror) {
+            Py_INCREF(self->winerror);
+            PyTuple_SET_ITEM(tuple, 0, self->winerror);
         }
         else {
             Py_INCREF(Py_None);