revlog: convert an Abort message to bytes

Flagged by pytype.

Differential Revision: https://phab.mercurial-scm.org/D10234

mercurial/revlog.py

@@ -3250,7 +3250,7 b' class revlog(object):'
                     # rewriting entries that already have sidedata is not
                     # supported yet, because it introduces garbage data in the
                     # revlog.
-                    msg = "Rewriting existing sidedata is not supported yet"
+                    msg = b"Rewriting existing sidedata is not supported yet"
                     raise error.Abort(msg)
                 entry = entry[:8]
                 entry += (current_offset, len(serialized_sidedata))