Allow #[classattr] on associated constants

[scalexm]

#[pymethods]
impl Foo {
    #[classattr]
    const MY_CONST: &'static str = "foobar";
}

[davidhewitt]

Very pleased to see this 👍

[davidhewitt]

I understand why you did this refactoring but I think I preferred how it was before, to keep symmetry with the the other impl_py_method_xyz functions.

[scalexm]

I hesitated making this change indeed. I’ll write two functions, it’ll duplicate code slightly but will preserve the symmetry.

[davidhewitt]

Also I think the original #905 missed off a CHANGELOG entry, so can you add one here?

(Might want to wait to rebase on #915 to make life easier.)

[kngwyu]

💯

[kngwyu]

Thank you!