wireprototypes: fix exception handling code with a bad pytype suppression

This goes back to f5fcf7123a92, and I suspect it was a misread of the error
message is describes- the LHS of `.sorted()` is always bytes, and bytes didn't
have this method in py2 either.  The invalid names were already handled like
this a few lines above.

PyCharm flagged this, and it stood out after converting the zope interfaces to
Protocol classes (which hasn't been published yet).

mercurial/wireprototypes.py

@@ -396,7 +396,7 def supportedcompengines(ui, role):
             )
             % config,
             hint=_(b'usable compression engines: %s')
-            % b', '.sorted(validnames),  # pytype: disable=attribute-error
+            % b', '.join(sorted(validnames)),
         )
 
     return compengines