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/json/encoder.py b/Lib/json/encoder.py
index 2d7b898..c8c78b9 100644
--- a/Lib/json/encoder.py
+++ b/Lib/json/encoder.py
@@ -268,7 +268,7 @@
         list=list,
         str=str,
         tuple=tuple,
-        _intstr=int.__str__,
+        _intstr=int.__repr__,
     ):
 
     if _indent is not None and not isinstance(_indent, str):
@@ -307,7 +307,7 @@
             elif value is False:
                 yield buf + 'false'
             elif isinstance(value, int):
-                # Subclasses of int/float may override __str__, but we still
+                # Subclasses of int/float may override __repr__, but we still
                 # want to encode them as integers/floats in JSON. One example
                 # within the standard library is IntEnum.
                 yield buf + _intstr(value)