PY3: "ZopeGuards" forgets to allow iteration over the result of "dict.{keys,values,items}"

[d-maurer]

Under Python 3, the nexts in the following code all result in an AccessControl.unauthorized.Unauthorized: You are not allowed to access 'a particular tuple' in this context:

from AccessControl.ZopeGuards import SafeIter, Containers

d={1:1, 2:2}
i=SafeIter(d.items())
next(i)
ik=SafeIter(d.keys())
next(ik)
iv=SafeIter(d.values())
next(iv)

This is the cause of zopefoundation/Products.PythonScripts#35.