Subclass built-in scalars without creating new GraphQL type

**Is your feature request related to a problem? Please describe.**
I'd like to be able to customise serialization logic for some scalars, e.g. a `NormalizedDecimal` that inherits from `Decimal` but calls `.normalize()` on the value during serialization. Unfortunately, this results in the GraphQL schema containing a new `NormalizedDecimal` scalar that clients don't know how to deserialize.

**Describe the solution you'd like**
I'd like the type to still be that of the superclass in the schema, e.g. `Decimal` in the above example.

**Describe alternatives you've considered**
I've tried overriding `Scalar.get_type()` to return the type of the superclass, but then the subclass doesn't function as intended, e.g. the custom serialization logic doesn't get invoked. I've also tried setting `Meta.name` to that of the superclass, but that also results in the custom serialization logic not being invoked.