convert: use bytes for value in extra dict

The keys and values of the extra dict are expected to be bytes. Before this
fix, there was a crash in mercurial.changelog.encodeextra().

hgext/convert/monotone.py

@@ -337,7 +337,7 b' class monotone_source(common.converter_s'
         extra = {}
         certs = self.mtngetcerts(rev)
         if certs.get(b'suspend') == certs[b"branch"]:
-            extra[b'close'] = 1
+            extra[b'close'] = b'1'
         dateformat = b"%Y-%m-%dT%H:%M:%S"
         return common.commit(
             author=certs[b"author"],