[BUG] cannot pass random.random() as std::function<double()> argument

Issue description

When passing random.random() as an argument while testing PyArmadillo, a RuntimeError is caused whenever the argument is called (see below). However, if the function is modified to accept py::function instead, the function works as intended. If the function is called through a lambda function, the function works as well. Functions defined by the user work fine.

It appears that passing a function of the Python class builtin_function_or_method as opposed to function does not work.

Expected behaviour: passing random.random as a std::function<> should work (i.e. the function should be callable and the return value should be available in C++)

Actual behaviour: throws a RuntimeError: Unable to extract capsule contents!

Reproducible example code

pybindtest.cpp

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

namespace py = pybind11;
using namespace std;

PYBIND11_MODULE(pybindtest, m) {
    m.def("foo", [](const std::function<double()> &f) { return f(); })
    .def("bar", [](const py::function &f) { return f(); });
}

pybindtest.py

from pybindtest import *
from random import random

def baz():
    return 123.456

# These work as intended
foo(baz)
foo(lambda: random())
foo(lambda: baz())

bar(random)
bar(baz)
bar(lambda: random())
bar(lambda: baz())

# RuntimeError: Unable to extract capsule contents!
foo(random)

Start with the std::function conversion path included from pybind11/functional.h, using the pybindtest.cpp and pybindtest.py reproducer. Compare the handling of random.random with user-defined functions and lambdas. Done means foo(random) is callable from Python without the capsule extraction RuntimeError and returns the function result.