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
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Ran 16 tests in 0.000s
OK