* Remove use of simplejson throughout code base. Fixes#42761
* Address failing tests
* Remove simplejson from contrib and other outlying files
* Add changelog fragment for simplejson removal
Replace .iteritems() with six.iteritems() everywhere except in
module_utils (because there's no 'six' on the remote host). And except
in lib/ansible/galaxy/data/metadata_template.j2, because I'm not sure
six is available there.