manifest: just duplicate the definition of items as iteritems

The forwarding trick was failing test-check-interfaces on Python
3. Duplicating a line of code is easy enough I'm doing that rather
than try and figure out what's going on in any kind of detail.

Differential Revision: https://phab.mercurial-scm.org/D3924

mercurial/manifest.py

@@ -534,7 +534,8 b' class manifestdict(object):'
     def items(self):
         return (x[:2] for x in self._lm.iterentries())
 
-    iteritems = items
+    def iteritems(self):
+        return (x[:2] for x in self._lm.iterentries())
 
     def iterentries(self):
         return self._lm.iterentries()