ResourceDictField.iteritems() and ResourceDictField.iterfields()

have been legacy for a long time, but with the removal of Python 2.7,

there's no longer any good reason to keep maintaining them.
This officially deprecates both. They will emit deprecation warnings on

use, recommending the modern replacements.
Other resource classes still have iter*() functions

(ItemResource.iterfields(), for example), but there's no replacement

function available, and we may have to worry about names clashing. This

change is ignoring this situation, leaving it to another day to solve.

Unit tests pass.