* 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()`.
