Include "object" as an argument type in the type signature of operator methods

Issue description

Currently when declaring an operator method, the type signature in the docstring is not changed to include the additional type. In the example below we see the docstring:

__eq__(self: my_class.MyClass, arg0: my_class.MyClass) -> bool

however I would expect it to include the additional type signature that is more generic:

__eq__(*args, **kwargs)
Overloaded function.

1. __eq__(self: my_class.MyClass, arg0: my_class.MyClass) -> bool

2. __eq__(self: my_class.MyClass, arg0: object) -> NotImplemented

The use case where this is coming up:
I'm generating type stubs of a pybind module using stubgen. When running mypy with the generated stubs I see the following error:

Argument 1 of "__eq__" is incompatible with supertype "object"; supertype defines the argument type as "object"

Reproducible example code

# include <pybind11/pybind11.h>

namespace py = pybind11;

class MyClass
{
    MyClass(int id) : m_id(id);
    int m_id;
};

PYBIND11_MODULE(my_class, m) {
    py::class_<MyClass>(m, "MyClass")
        .def("__eq__",
            [](const MyClass& self, const MyClass& other) {
                return self.m_id == other.m_id;
            },
            py::is_operator()
        )
    ;
}

>>> import my_class
>>> my_class.MyClass.__eq__.__doc__
'__eq__(self: my_class.MyClass, arg0: my_class.MyClass) -> bool\n'

Start by tracing how py::is_operator affects operator signatures and how pybind11 assembles the generated doc text; then compare the behavior with the stubgen and mypy example in the issue. Done means an operator such as eq exposes both the concrete argument signature and the additional object signature returning NotImplemented.