[BUG?] The __get__ function is missing from pybind11::cpp_function

[ianhbell]

I am adding bound methods to a class at runtime (#4311). The cpp_function class does not expose the __get__ method, so I cannot use the descriptor method (https://stackoverflow.com/a/28060251/1360263) to generate a bound method to the instance. It would be nice if that approach could be supported.