Fix non_local_definitions warnings from RFC 3373

[davidhewitt]

Follow up to #3901

I will leave FIXME places in the code where the #[allow(non_local_definitions)] lint is firing. Broadly I think this lint falls into two camps:

• We use const _: () = { use ::pyo3 as _pyo3; /* macro expanded code */ } to allow use-ing PyO3 at a consistent name across the PyO3 codebase. This was convenient to support the #[pyo3(crate = "...")] option. I think we could refactor the macros to not need this anonymous const, and that might solve the problem.
• Inside create_exception! and its inner macros we have implementations of traits for &MyCustomException, which the lint seems to not like. We may be able to remove these cases alongside the GIL Ref API.

[mejrs]

I'll have a crack at this