[BUG] Unbound methods should just be functions instead of instancemethod

Issue description

Objects of classes created with pybind11 can't be pretty printed with the pprint module if the result is larger than the given width.
This only happens with python3.

It used to happen with cython too, but cython stopped using 'instancemethod', which solved the issue. The pprint bug was discarded because "Functions/methods should be immutable, so yes, I think it is a safe assumption that they should be hashable":

https://bugs.python.org/issue33395

Reproducible example code

example.cpp:

#include <pybind11/pybind11.h>
#include <string>

namespace py = pybind11;

struct Dummy
{
  std::string text;
};

PYBIND11_MODULE(example, m) {
  using namespace pybind11::literals;

  py::class_<Dummy>(m, "Dummy")
      .def(py::init<std::string>(), "text"_a)
      .def_readonly("text", &Dummy::text)
      .def("__str__", [](const Dummy& w) { return w.text; })
      .def("__repr__", [](const Dummy& w) { return w.text; });
}

bug.py:

import example
import pprint

pprint.pprint(example.Dummy('abc'), width=3)

Result:

Traceback (most recent call last):
  File "bug.py", line 4, in <module>
    pprint.pprint(example.Dummy('abc'), width=2)
  File "/usr/lib/python3.8/pprint.py", line 53, in pprint
    printer.pprint(object)
  File "/usr/lib/python3.8/pprint.py", line 148, in pprint
    self._format(object, self._stream, 0, 0, {}, 0)
  File "/usr/lib/python3.8/pprint.py", line 173, in _format
    p = self._dispatch.get(type(object).__repr__, None)
TypeError: unhashable type: 'instancemethod'

Start by building the binding shown in example.cpp and running bug.py with Python 3 to reproduce the pprint TypeError. Read the pybind11 class and method-binding entry points related to unbound methods, then verify that pprint.pprint handles the resulting object at a narrow width without raising the unhashable instancemethod error.