Fix warning about casting incompatible function types #308

Debian 10 (Buster) has gcc 8.2 which warns about:

cast between incompatible function types from ‘PyObject * (*)(PyObject *, PyObject *, PyObject *)’

The solution is to cast to '(PyCFunction)(void *)'

Author: pdp7

source/py_gpio.c

@@ -555,14 +555,14 @@ static PyObject *py_setwarnings(__attribute__ ((unused)) PyObject *self, __attri
 static const char moduledocstring[] = "GPIO functionality of a BeagleBone using Python";
 
 PyMethodDef gpio_methods[] = {
-   {"setup", (PyCFunction)py_setup_channel, METH_VARARGS | METH_KEYWORDS, "Set up the GPIO channel, direction and (optional) pull/up down control\nchannel        - Either: RPi board pin number (not BCM GPIO 00..nn number).  Pins start from 1\n                 or    : BCM GPIO number\ndirection      - INPUT or OUTPUT\n[pull_up_down] - PUD_OFF (default), PUD_UP or PUD_DOWN\n[initial]      - Initial value for an output channel\n[delay]        - Time in milliseconds to wait after exporting gpio pin"},
+   {"setup", (PyCFunction)(void *)py_setup_channel, METH_VARARGS | METH_KEYWORDS, "Set up the GPIO channel, direction and (optional) pull/up down control\nchannel        - Either: RPi board pin number (not BCM GPIO 00..nn number).  Pins start from 1\n                 or    : BCM GPIO number\ndirection      - INPUT or OUTPUT\n[pull_up_down] - PUD_OFF (default), PUD_UP or PUD_DOWN\n[initial]      - Initial value for an output channel\n[delay]        - Time in milliseconds to wait after exporting gpio pin"},
    {"cleanup", py_cleanup, METH_VARARGS, "Clean up by resetting all GPIO channels that have been used by this program to INPUT with no pullup/pulldown and no event detection"},
    {"output", py_output_gpio, METH_VARARGS, "Output to a GPIO channel\ngpio  - gpio channel\nvalue - 0/1 or False/True or LOW/HIGH"},
    {"input", py_input_gpio, METH_VARARGS, "Input from a GPIO channel.  Returns HIGH=1=True or LOW=0=False\ngpio - gpio channel"},
-   {"add_event_detect", (PyCFunction)py_add_event_detect, METH_VARARGS | METH_KEYWORDS, "Enable edge detection events for a particular GPIO channel.\nchannel      - either board pin number or BCM number depending on which mode is set.\nedge         - RISING, FALLING or BOTH\n[callback]   - A callback function for the event (optional)\n[bouncetime] - Switch bounce timeout in ms for callback"},
+   {"add_event_detect", (PyCFunction)(void *)py_add_event_detect, METH_VARARGS | METH_KEYWORDS, "Enable edge detection events for a particular GPIO channel.\nchannel      - either board pin number or BCM number depending on which mode is set.\nedge         - RISING, FALLING or BOTH\n[callback]   - A callback function for the event (optional)\n[bouncetime] - Switch bounce timeout in ms for callback"},
    {"remove_event_detect", py_remove_event_detect, METH_VARARGS, "Remove edge detection for a particular GPIO channel\ngpio - gpio channel"},
    {"event_detected", py_event_detected, METH_VARARGS, "Returns True if an edge has occured on a given GPIO.  You need to enable edge detection using add_event_detect() first.\ngpio - gpio channel"},
-   {"add_event_callback", (PyCFunction)py_add_event_callback, METH_VARARGS | METH_KEYWORDS, "Add a callback for an event already defined using add_event_detect()\ngpio         - gpio channel\ncallback     - a callback function\n[bouncetime] - Switch bounce timeout in ms"},
+   {"add_event_callback", (PyCFunction)(void *)py_add_event_callback, METH_VARARGS | METH_KEYWORDS, "Add a callback for an event already defined using add_event_detect()\ngpio         - gpio channel\ncallback     - a callback function\n[bouncetime] - Switch bounce timeout in ms"},
    {"wait_for_edge", py_wait_for_edge, METH_VARARGS, "Wait for an edge.\ngpio - gpio channel\nedge - RISING, FALLING or BOTH\ntimeout (optional) - time to wait in miliseconds. -1 will wait forever (default)"},
    {"gpio_function", py_gpio_function, METH_VARARGS, "Return the current GPIO function (IN, OUT, ALT0)\ngpio - gpio channel"},
    {"setwarnings", py_setwarnings, METH_VARARGS, "Enable or disable warning messages"},