Adding PyGILState_Check() in object_api<>::operator(). (#2919)

* Adding PyGILState_Check() in object_api<>::operator().

* Enabling PyGILState_Check() for Python >= 3.6 only.

Possibly, this explains why PyGILState_Check() cannot safely be used with Python 3.4 and 3.5:

https://github.com/python/cpython/pull/10267#issuecomment-434881587

* Adding simple micro benchmark.

* Reducing test time to minimum (purely for coverage, not for accurate results).

* Fixing silly oversight.

* Minor code organization improvement in test.

* Adding example runtimes.

* Removing capsys (just run with `-k test_callback_num_times -s` and using `.format()`.

tests/test_callbacks.cpp

@@ -172,4 +172,10 @@ TEST_SUBMODULE(callbacks, m) {
         for (auto i : work)
             start_f(py::cast<int>(i));
     });
+
+    m.def("callback_num_times", [](py::function f, std::size_t num) {
+        for (std::size_t i = 0; i < num; i++) {
+            f();
+        }
+    });
 }