bindings: python: fix GCC8 warnings due to function pointer casting

bindings: python: fix GCC8 warnings due to function pointer casting

GCC8 has added a new warning heuristic to detect invalid function
casts. This causes a lot of warnings when building libgpiod python
bindings.

Let's use Py_UNUSED where applicable to make function prototypes match
that of PyCFunction. For keyword functions, let's cast them to a
function pointer taking no arguments before casting them again to
PyCFunction. This may not be the best solution but it has to do for
the moment.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>