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/xmlrpc/client.py b/Lib/xmlrpc/client.py
index a0e923a..b987574 100644
--- a/Lib/xmlrpc/client.py
+++ b/Lib/xmlrpc/client.py
@@ -186,8 +186,7 @@
 
 class Error(Exception):
     """Base class for client errors."""
-    def __str__(self):
-        return repr(self)
+    __str__ = object.__str__
 
 ##
 # Indicates an HTTP-level protocol error.  This is raised by the HTTP
@@ -869,8 +868,6 @@
     def __repr__(self):
         return "<%s at %#x>" % (self.__class__.__name__, id(self))
 
-    __str__ = __repr__
-
     def __getattr__(self, name):
         return _MultiCallMethod(self.__call_list, name)
 
@@ -1468,8 +1465,6 @@
             (self.__class__.__name__, self.__host, self.__handler)
             )
 
-    __str__ = __repr__
-
     def __getattr__(self, name):
         # magic method dispatcher
         return _Method(self.__request, name)