Structs impling Default should have default Python constructors

Currently pyo3 bindings require you to explicitly annotate any Rust "constructor" as a Python constructor. This means that if your struct relies on its default() method from the Default trait you cannot expose the default() method directly. Instead you have to do this:

#[pymethods]
impl MyStruct {
    #[new]
    fn new() -> Self {
        Self::default()
    }
}

In my opinion, exposing this default() method is a common enough problem that this boilerplate is a little unnecessary. What if instead we could do

#[derive(Default)]
#[pyclass(default)]
struct MyStruct;

Here the pyclass(default) would create a zero-argument constructor method for the Python type, and its implementation would call MyStruct::default.

Start by tracing the #[pyclass] handling and the existing #[pymethods] constructor behavior described in the issue. Verify how a #[pyclass(default)] option could call the Rust Default implementation and expose a zero-argument Python constructor; done means the proposed annotation works for a #[derive(Default)] struct and preserves current constructor behavior.