Replace `_self` by `self` when the feature `arbitrary_self_types` is merged

[FirelightFlagboy]

error[E0658]: `pyo3::PyRef<'_, organization::OrganizationStatsRepOk>` cannot be used as the type of `self` without the `arbitrary_self_types` feature                                                   
   --> src/protocol/organization.rs:135:24
    |
135 |     fn data_size(self: PyRef<'_, Self>) -> PyResult<u64> {
    |                        ^^^^^^^^^^^^^^^
    |
    = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
    = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)

Originally posted by @TimeEngineer in https://github.com/Scille/parsec-cloud/pull/3016#discussion_r963463335

Require:

• rust-lang/rust#44874