Add experimental iterkeys(), itervalues(), iteritems() to dict objects.
Add experimental iterkeys(), itervalues(), iteritems() to dict
objects.

Tests show that iteritems() is 5-10% faster than iterating over the
dict and extracting the value with dict[key].

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator