fix TypeError: function takes exactly 3 arguments (2 given) from wait_for_edge

Trying to parse 3 required arguments and then falling back to 2 will set the
PyError condition when only two arguments are present even though the
wait_for_edge function as a whole succeeds.  According to
https://docs.python.org/2/c-api/exceptions.html, returning normally after this
will lead to undefined behavior the next time the code enters the C API.

Fix this by using the PyArg_ParseTuple optional argument feature instead of
trying to parse 3 arguments and falling back to 2 if it fails.

Author: DJ Sutton

source/py_gpio.c

@@ -444,10 +444,9 @@ static PyObject *py_wait_for_edge(__attribute__ ((unused)) PyObject *self, PyObj
    char error[30];
    BBIO_err err;
 
-   if (!PyArg_ParseTuple(args, "sii", &channel, &edge, &timeout)){
-      timeout = -1;
-      if (!PyArg_ParseTuple(args, "si", &channel, &edge))
-         return NULL;
+   timeout = -1;
+   if (!PyArg_ParseTuple(args, "si|i", &channel, &edge, &timeout)){
+      return NULL;
    }
 
    err = get_gpio_number(channel, &gpio);