Reduce the max stack depth to see if this fixes the segfaults on

Windows and some other boxes.  If this is successful, this rev should
be backported.  I'm not sure how close to the limit we should push this.

Python/marshal.c

@@ -15,7 +15,7 @@
  * and risks coring the interpreter.  When the object stack gets this deep,
  * raise an exception instead of continuing.
  */
-#define MAX_MARSHAL_STACK_DEPTH 5000
+#define MAX_MARSHAL_STACK_DEPTH 4000
 
 #define TYPE_NULL		'0'
 #define TYPE_NONE		'N'