linelog: suppress annoying pytype warning about an ignored metaclass

This isn't actively hurting us right now, and it's not trivial to set
up this metaclass correctly on both Python 2 and Python 3 without
dragging in all of `six`, but we still get lots of typechecking
goodness with this error suppressed.

Differential Revision: https://phab.mercurial-scm.org/D7270

mercurial/linelog.py

@@ -53,7 +53,7 b' class annotateresult(object):'
         return iter(self.lines)
 
 
-class _llinstruction(object):
+class _llinstruction(object):  # pytype: disable=ignored-metaclass
 
     __metaclass__ = abc.ABCMeta