py3: do not reimplement Abort.__str__() on Python 2

It isn't necessary on Python 2, and the default implementation should be
better than our BaseException_str() clone.

mercurial/error.py

@@ -111,8 +111,10 b' class Abort(Hint, Exception):'
 
     __bytes__ = _tobytes
 
-    def __str__(self):
-        return pycompat.sysstr(self.__bytes__())
+    if pycompat.ispy3:
+
+        def __str__(self):
+            return pycompat.sysstr(self.__bytes__())
 
 
 class HookLoadError(Abort):