ui: fix Python 2.7 support for ui.timestamp-output

Differential Revision: https://phab.mercurial-scm.org/D8675

mercurial/ui.py

@@ -1229,7 +1229,8 b' class ui(object):'
         }
         if timestamp:
             args = (
-                b'[%s] ' % bytes(datetime.datetime.now().isoformat(), 'ASCII'),
+                b'[%s] '
+                % pycompat.bytestr(datetime.datetime.now().isoformat()),
             ) + args
         _writemsgwith(self._write, dest, *args, **opts)
         if timestamp: