typing: disable a module-attr warning in the worker module's py2 code

Differential Revision: https://phab.mercurial-scm.org/D10183

mercurial/worker.py

@@ -104,7 +104,9 b' if pycompat.ispy3:'
 else:
 
     def ismainthread():
+        # pytype: disable=module-attr
         return isinstance(threading.current_thread(), threading._MainThread)
+        # pytype: enable=module-attr
 
     def _blockingreader(wrapped):
         return wrapped