[QUESTION] Possibility to return py::object from py::init method

Hi,

Currently it seems to be impossible to return py::object from an init function. I'm wondering why this restriction is in place, and whether it could be removed, especially in the case where the class is marked with py::dynamic_attr. I could imagine an extra case where the underlying C++ type is checked to be of the correct type, i.e. compatible pointer, holder, or value.

Consider the following code:

class Dummy{};
py::class_<Dummy>( mod, "Dummy", py::dynamic_attr() )
        //.def( py::init( []() -> Dummy {return Dummy();})) // OK
        .def( py::init( []() -> py::object {
            py::object dummy = py::cast(Dummy());
            setattr(dummy, "foo", py::int_(1));
            return dummy;
        })) // Does not compile
        ;

With error:
pybind11\detail\init.h(86,48): error C2338: pybind11::init(): init function must return a compatible pointer, holder, or value

The equivalent python code works without issue

class Dummy:
    def __init__(self):
        setattr(self, "foo", 1)

d = Dummy() # no errors here

Start by reading the init validation in pybind11/detail/init.h, especially the assertion requiring a compatible pointer, holder, or value, and compare it with the py::dynamic_attr example in the issue. Determine the supported behavior and type-safety requirements for returning py::object, then add focused coverage demonstrating the intended constructor behavior.