Check if lookup can operates on select

To get decrypted values we need to run a SQL like this one:

```SQL
SELECT pgp_pub_decrypt(fieldname, private_key) FROM table;
```

We might be able to achieve the same thing as the `annotate` with django lookup.

cc @maxpeterson 
If we're monkey-patching in `HashedTextField.get_lookup` then this line is unnecessary.