Trait_bounds example should not need to acquire GIL

[davidhewitt]

In the trait_bounds example, the trait signature impl Model for UserModel has no way to pass a Python into the trait methods. The idea for this example is to show how to wrap a Rust trait which doesn't know anything about Python, so I don't think changing the trait definition is a solution either.

Originally posted by @davidhewitt in #1037

It would be nice to come up with a pattern here which allows #[pyclass] structs which work like in this example to implement Rust traits without needing to acquire GIL.

The trait doesn't even have to be a complicated one like in the trait_bounds example. For some #[pyclass] structs even their Display implementation may need to acquire the GIL.