Trait_bounds example should not need to acquire GIL

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 https://github.com/PyO3/pyo3/pull/1037/review_comment/create

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.

Start by reading the trait_bounds example and the surrounding discussion about #[pyclass] trait implementations and GIL acquisition. Determine a pattern that lets Rust traits such as Display work without changing the trait definition; done means the example and relevant behavior no longer require acquiring the GIL unnecessarily.