Unsupported query: mixed type addition in SELECT

For example:
```
======================================================================
ERROR: test_mixed_type_annotation_numbers (annotations.tests.NonAggregateAnnotationTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/home/tim/code/django/django/db/backends/utils.py", line 84, in _execute
return self.cursor.execute(sql, params)
psycopg2.errors.InvalidParameterValue: unsupported binary operator: +
```
Is there a plan to add support?

The SQL adds integer and float columns in a `SELECT` clause:
```sql
SELECT ... ("annotations_book"."pages" + "annotations_book"."rating") AS "combined"
FROM "annotations_book"
```