py3: use bytes() instead of str()

Differential Revision: https://phab.mercurial-scm.org/D2665

mercurial/revlog.py

@@ -2468,7 +2468,7 b' class revlog(object):'
                 if populatecachedelta:
                     dp = self.deltaparent(rev)
                     if dp != nullrev:
-                        cachedelta = (dp, str(self._chunk(rev)))
+                        cachedelta = (dp, bytes(self._chunk(rev)))
 
                 if not cachedelta:
                     rawtext = self.revision(rev, raw=True)