[1.8.x] Fixed a query failure on Python 3.5; refs #23763.

The failure was introduced in Django by
c7fd9b2 and the change in
Python 3.5 is https://hg.python.org/cpython/rev/a3c345ba3563.

Backport of be1357e from master

django/db/models/sql/query.py

@@ -402,7 +402,7 @@ def get_aggregation(self, using, added_aggregate_names):
             # Remove any aggregates marked for reduction from the subquery
             # and move them to the outer AggregateQuery.
             col_cnt = 0
-            for alias, expression in inner_query.annotation_select.items():
+            for alias, expression in list(inner_query.annotation_select.items()):
                 if expression.is_summary:
                     expression, col_cnt = inner_query.rewrite_cols(expression, col_cnt)
                     outer_query.annotations[alias] = expression.relabeled_clone(relabels)