Support `PyTypeObject`

[asadchev]

I couldnt find if PyBind supports type objects. If not, then something like this is useful:

  struct typeobject : py::handle {
    typeobject(PyTypeObject *type)
      : py::handle((PyObject*)type)
    {
    }
    typeobject static float_() { return typeobject(&PyFloat_Type); }
    typeobject static int_() { return typeobject(&PyInt_Type); }
    typeobject static str() { return typeobject(&PyString_Type); }
  };

or

  py::handle typeobject(PyTypeObject *type) {
    return py::handle((PyObject*)type);
  }

  template<typename T>
  py::handle typeobject();

  template<>
  py::handle typeobject<py::float_>() { return typeobject(&PyFloat_Type); }

  template<>
  py::handle typeobject<py::int_>() { return typeobject(&PyInt_Type); }

  template<>
  py::handle typeobject<py::str>() { return typeobject(&PyString_Type); }

for example handling type arguments
module.def("foo", py::arg("bar") = typeobject::str());