Abstract classes and methods are not abstract after wrapping

Issue description

In C++, abstract classes are defined as classes that contain pure virtual methods. Python's equivalent has been introduced in PEP -3119 – Abstract Base Classes.
Both cases represent the same concept: Types that cannot be instantiated and merely define a generic interface.

pybind11 already knows the notion of pure virtual functions and provides PYBIND11_OVERLOAD_PURE. In order to be compatible with python, classes containing such methods should be registered as Abstract Base Classes, the respective methods should be decorated with @abstractmethod.

Reproducible example code

C++

#include <pybind11/pybind11.h>

namespace py = pybind11;

class Abstract {
public:
	virtual ~Abstract() = default;

	virtual void some_method() = 0;
};


class PyAbstract : public Abstract {
public:
	using Abstract::Abstract;

	void some_method() override {
		PYBIND11_OVERLOAD_PURE(void, Abstract, some_method)
	}
};

PYBIND11_MODULE(abc_example, m) {
	py::class_<Abstract, PyAbstract>(m, "Abstract")
			.def(py::init<>())
			.def("some_method", &Abstract::some_method);
}

Python test

import unittest
import inspect
from abc_example import Abstract

class AbstractBaseTest(unittest.TestCase):
    def test_is_abstract(self):
        self.assertTrue(inspect.isabstract(Abstract))

    def test_abstract_method(self):
        self.assertTrue(
            getattr(Abstract.some_method, "__isabstractmethod__", False)
        )

if __name__ == '__main__':
    unittest.main()

With the most recent version of pybind11 (tested: 2.4.3), both tests fail.

Start with the py::class_ registration and PYBIND11_OVERLOAD_PURE usage shown in the C++ example, then compare them with Python's abc and inspect.isabstract behavior. Use the provided AbstractBaseTest checks as the acceptance criteria: the wrapped class must be recognized as abstract and its bound method must expose isabstractmethod.