Unsupported query: sum(): unsupported binary operator: <float> + <int>

Example query:
```sql
SELECT ... sum((("aggregation_book"."rating" + "aggregation_book"."pages") + "aggregation_book"."price")) AS "sums"
FROM "aggregation_book"
WHERE "aggregation_book"."id" = 501322346762469377
GROUP BY "aggregation_book"."id";
```
Where `rating` is `FloatField` and `pages` is `IntegerField`.

Django test failure:
```
======================================================================
ERROR: test_add_implementation (aggregation.tests.AggregateTestCase)
----------------------------------------------------------------------
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: sum(): unsupported binary operator: +

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/home/tim/code/django/tests/aggregation/tests.py", line 1045, in test_add_implementation
b1 = qs.get(pk=self.b4.pk)
File "/home/tim/code/django/django/db/models/query.py", line 402, in get
num = len(clone)
File "/home/tim/code/django/django/db/models/query.py", line 256, in __len__
self._fetch_all()
File "/home/tim/code/django/django/db/models/query.py", line 1242, in _fetch_all
self._result_cache = list(self._iterable_class(self))
File "/home/tim/code/django/django/db/models/query.py", line 55, in __iter__
results = compiler.execute_sql(chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size)
File "/home/tim/code/django/django/db/models/sql/compiler.py", line 1100, in execute_sql
cursor.execute(sql, params)
File "/home/tim/code/django/django/db/backends/utils.py", line 99, in execute
return super().execute(sql, params)
File "/home/tim/code/django/django/db/backends/utils.py", line 67, in execute
return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
File "/home/tim/code/django/django/db/backends/utils.py", line 76, in _execute_with_wrappers
return executor(sql, params, many, context)
File "/home/tim/code/django/django/db/backends/utils.py", line 84, in _execute
return self.cursor.execute(sql, params)
File "/home/tim/code/django/django/db/utils.py", line 89, in __exit__
raise dj_exc_value.with_traceback(traceback) from exc_value
File "/home/tim/code/django/django/db/backends/utils.py", line 84, in _execute
return self.cursor.execute(sql, params)
django.db.utils.DataError: sum(): unsupported binary operator: +

```