py3: use bytes() to cast to immutable bytes in changelog.appender.write()

mercurial/changelog.py

@@ -120,7 +120,7 b' class appender(object):'
         return ret
 
     def write(self, s):
-        self.data.append(str(s))
+        self.data.append(bytes(s))
         self.offset += len(s)
         self._end += len(s)