obsolete: use bytes() instead of str() so the node is bytes on py3

I'm not sure this is right, since this should either be bytes or str
to match what's going on in the revlog layer.

Differential Revision: https://phab.mercurial-scm.org/D271

mercurial/obsolete.py

@@ -583,7 +583,7 b' class obsstore(object):'
 
         metadata = tuple(sorted(metadata.iteritems()))
 
-        marker = (str(prec), tuple(succs), int(flag), metadata, date, parents)
+        marker = (bytes(prec), tuple(succs), int(flag), metadata, date, parents)
         return bool(self.add(transaction, [marker]))
 
     def add(self, transaction, markers):