wireprototypes: disable pytype where it's just confused

By inspection validnames is always a Set[bytes] here, but for some
reason pytype is convinced it's a bytes. Let's disable the error for now.

Differential Revision: https://phab.mercurial-scm.org/D7281

mercurial/wireprototypes.py

@@ -404,7 +404,7 b' def supportedcompengines(ui, role):'
             )
             % config,
             hint=_(b'usable compression engines: %s')
-            % b', '.sorted(validnames),
+            % b', '.sorted(validnames),  # pytype: disable=attribute-error
         )
 
     return compengines