py3: don't use dict.iterkeys()

Instead of iterating of dict.iterkeys(), let's iterate of dict as that will be
equivalent and dict.iterkeys() is not present in Python 3.

Differential Revision: https://phab.mercurial-scm.org/D1796

mercurial/manifest.py

@@ -810,7 +810,7 b' class treemanifest(object):'
             if p in self._files:
                 yield self._subpath(p)
             else:
-                for f in self._dirs[p].iterkeys():
+                for f in self._dirs[p]:
                     yield f
 
     def keys(self):