bpo-36793: Remove unneeded __str__ definitions. (GH-13081)

Classes that define __str__ the same as __repr__ can
just inherit it from object.
diff --git a/Lib/sre_constants.py b/Lib/sre_constants.py
index 13deb00..8e613cb 100644
--- a/Lib/sre_constants.py
+++ b/Lib/sre_constants.py
@@ -59,11 +59,9 @@
         self.name = name
         return self
 
-    def __str__(self):
+    def __repr__(self):
         return self.name
 
-    __repr__ = __str__
-
 MAXREPEAT = _NamedIntConstant(MAXREPEAT, 'MAXREPEAT')
 
 def _makecodes(names):