Add support for #[pyclass(get(attr1, attr2, ...))] and #[pyclass(set(attr1, attr2, ...))]

[davidhozic]

Solves the problem in #5125, which mentions the problem of having helper attributes, such as #[pyo3(get)], while conditionally enabling pyclass using cfg_attr.

The problem is solved by allowing #[pyclass] to have options get and set, which accept a comma separated sequence of attribute names.

Progress:

• Implement #[pyclass(get(attr1, attr2, ...))];
• Implement #[pyclass(set(attr1, attr2, ...))];
• Document changes;
• Update documentation.

Example

#[cfg_attr(feature = "python-bindings", pyclass(module = "game", get(score, running), set(running)))]
pub struct GameManager {
    score: [u16; 2],
    running: bool
}

[davidhozic]

Note: the tests in question seem to fail on the main branch as well.