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

@@ -57,15 +57,17 @@ struct type_caster<std::function<Return(Args...)>> {
         // ensure GIL is held during functor destruction
         struct func_handle {
             function f;
-            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);
             }
         };
 
-        value = [hfunc = func_handle(std::move(func))](Args... args) -> Return {
+        value = [hfunc = func_handle{func}](Args... args) -> Return {
             gil_scoped_acquire acq;
             object retval(hfunc.f(std::forward<Args>(args)...));
             /* Visual studio 2015 parser issue: need parentheses around this expression */