error: unconditionally define __str__

We always run on Python 3 now.

Differential Revision: https://phab.mercurial-scm.org/D12292

mercurial/error.py

@@ -68,8 +68,6 b' class Error(Hint, Exception):'
     def __bytes__(self):
         return self.message
 
-    if pycompat.ispy3:
-
         def __str__(self):
             # type: () -> str
             # the output would be unreadable if the message was translated,