Use native API for holding GIL when Python functor is being destructed

Don't use `gil_scoped_aquire` because it can result in deadlock.

Author: uentity

include/pybind11/functional.h

@@ -60,8 +60,12 @@ struct type_caster<std::function<Return(Args...)>> {
             func_handle(function&& f_) : f(std::move(f_)) {}
             func_handle(const func_handle&) = default;
             ~func_handle() {
-                gil_scoped_acquire acq;
-                function kill_f(std::move(f));
+                PyGILState_STATE gstate;
+                gstate = PyGILState_Ensure();
+                {
+                    function kill_f(std::move(f));
+                }
+                PyGILState_Release(gstate);
             }
         };