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

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.

--- a/mercurial/error.py	Mon Nov 04 20:57:31 2019 -0800
+++ b/mercurial/error.py	Tue Nov 05 21:29:40 2019 +0900
@@ -111,8 +111,10 @@
 
     __bytes__ = _tobytes
 
-    def __str__(self):
-        return pycompat.sysstr(self.__bytes__())
+    if pycompat.ispy3:
+
+        def __str__(self):
+            return pycompat.sysstr(self.__bytes__())
 
 
 class HookLoadError(Abort):