fix pydantic return type serializer to support lists

[thrau]

Motivation

I forgot to handle lists of pydantic models in #14. You can now do something like:

@route("/items")
def handler(_request: Request) -> list[MyItem]:
    return [
        MyItem(name="rolo", ...),
        MyItem(name="twiks", ...),
    ]

Changes

• lists of pydantic models can now be returned from a handler using a handler_dispatcher