Robust test for callable objects

[DimitriPapadopoulos]

• If the object implements a custom __getattr__, or if its __call__ is itself not callable, you may get misleading results.
• Instead use the built-in callable function. Not only is it more robust, but it makes the intent clear and the code more readable. Requires https://github.com/astanin/python-tabulate/pull/373.