* Use the new bound API instead of .as_ref(py)
* Move import into a nested scope
* Use to_cow instead of to_str for compatibility
`to_str` is not available before Python 3.10 on the limited api.
* Relax &self lifetimes
* Use Bound<'py, PyAny> in test Mapping signatures
* Use .as_bytes(py)
* Simplify ThrowCallback::throw signature
* Avoid .as_any call with Py api instead of Bound