fix: memory leak in cpp_function (#3228) (#3229)

* fix: memory leak in cpp_function (#3228)

* add a test case to check objects are deconstructed in cpp_function

* update the test case about cpp_function

* fix the test case about cpp_function: remove "noexcept"

* Actually calling func. CHECK(stat.alive() == 2); Manually verified that the new tests fails without the change in pybind11.h

* Moving new test to test_callbacks.cpp,py, with small enhancements.

* Removing new test from test_interpreter.cpp (after it was moved to test_callbacks.cpp,py). This restores test_interpreter.cpp to the current state on master.

* Using py::detail::silence_unused_warnings(py_func); to make the intent clear.

Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>