COW.py: fix sample codes

COW.py: fix sample codes

The call of methods iteritems() and itervalues() in sample codes were
replaced by items() and values() to convert to Python 3 by Bitbake rev
d0f904d407f57998419bd9c305ce53e5eaa36b24. But the methods iteritems()
and itervalues() belong to class COWDictMeta not class dict or set. The
modifications should not be made in purpose that it fails to run sample
codes, so revert them.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>