Regression using Self inside #[pymethods] arguments

[davidhewitt]

This snippet used to work on pyo3 0.8:

#[pyclass]
struct MyClass { }

#[pymethods]
impl MyClass {
    fn takes_self(&self, other: &Self) { }
}

With 0.11.1 it fails with:

error[E0411]: cannot find type `Self` in this scope=================>  ] 35/36: ...
  --> src/lib.rs:14:34
   |
14 |     fn takes_self(&self, other: &Self) { }
   |                                  ^^^^ `Self` is only available in impls, traits, and type definitions

This was originally reported in #791 (comment), which also identifies possible guilty line.