Using subclass of Range as default return type

* **asyncpg version**: 0.20.1
* **PostgreSQL version**: 12.1
* **Python version**: 3.7.7
* **Platform**: macOS 10.15.4
* **Do you use pgbouncer?**: No
* **Did you install asyncpg with pip?**: Yes

I created a subclass of the Range type as follows:
```
from asyncpg.types import Range as Rg

class Range(Rg):
    def issubset(self, other):
        return self._lower >= other._lower and self._upper <= other._upper

    def issuperset(self, other):
        return self._upper >= other._upper and self._lower <= other._lower

    def size(self):
        return self._upper - self._lower

```

Would it be possible to have this subclass used automatically instead of the default type?

Both `set_type_codec` and `set_builtin_type_codec` don't seem to be able to do it.



Description