casting c++-base class to py-derived class failes

Issue description

Hello,
Consider a derived py-class (PClass) with a C++ class as base class (CClass).
If I want cast a CClass-Object to a PClass-Object, python gives me the following error:
TypeError: class assignment: 'PClass' deallocator differs from 'cpp_module.CClass'

I tested this with python3, clang-6.0, gcc 5.4, c++11 and c++17

Reproducible example code

#include <pybind11/pybind11.h>

class CClass {};

namespace py = pybind11;

PYBIND11_MODULE(cpp_module, m) {
   py::class_<CClass>(m, "CClass").def(py::init<>());
}

working example with python

class PClassA():
    def __init__(self):
        pass

class PClassB(PClassA):
    def __init__(self):
        super().__init__()

# py -> py example
a_class = PClassA()
b_class = PClassB()
a_class.__class__ = PClassB # this works

failing example with C++

from cpp_module import CClass

class PClass(CClass):
    def __init__(self):
        super().__init__()

# c++ -> py example
c_class = CClass()
p_class = PClass()
c_class.__class__ = PClass  # this failes

output

Traceback (most recent call last):
  File "test.py", line 25, in <module>
    c_class.__class__ = PClass  # this failes
TypeError: __class__ assignment: 'PClass' deallocator differs from 'cpp_module.CClass'

The reproducible entry point is the CClass binding in pybind11/pybind11.h; first run the supplied C++ and Python examples to confirm the class assignment failure. Done means assigning CClass.class to PClass no longer raises the reported deallocator error.