At the moment the protocol methods are in an inconsistent state: some of them can take PyRef or PyRefMut, and some of them take &self or &mut self.

This is confusing to users and also gets in the way in certain cases like needing to access Python inside a protocol method (which can be obtained from PyRef::py() for example) or wanting to return PyRef<Self>.

The simple solution is to just change all protocol methods to use TryFromPyCell trait. This is however a breaking change.

The better solution is to change all #[pyproto] methods to support any of the five of &PyCell, PyRef<Self>, PyRefMut<Self>, &self and &mut self, just like we support for #[pymethods].

I've had some ideas how to approach this second point so would like to take a shot at it soon.