* 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
The methods are not expected by CPython and are only explicitly
callable. To get iteration support, PyIterProtocol should be
implemented and to get support for `x in mapping`,
PySequenceProtocol's __contains__ should be implemented.
https://github.com/PyO3/pyo3/issues/611
