Change iteritems to items for Python 3

templates/.composer/auth.json.j2

 {
     {% if composer_http_basic %}
     "http-basic": {
-        {% for repository,credentials in composer_http_basic.iteritems() %}
+        {% for repository,credentials in composer_http_basic.items() %}
         {% if loop.index0 %},{% endif %}
         "{{ repository }}": {
             "username": "{{ credentials.username }}",