[Python] Fix new GCC8 warnings in generated code

Avoid casts between incompatible function types where possible (when
keyword args are in use, it is not possible to avoid such warnings as
they are inherent in the design of Python's C API in that particular
case).  Fixes #1259.

Lib/python/pyinit.swg

@@ -347,8 +347,8 @@ SWIG_init(void) {
     (char *)"this", &SwigPyBuiltin_ThisClosure, NULL, NULL, NULL
   };
   static SwigPyGetSet thisown_getset_closure = {
-    (PyCFunction) SwigPyObject_own,
-    (PyCFunction) SwigPyObject_own
+    SwigPyObject_own,
+    SwigPyObject_own
   };
   static PyGetSetDef thisown_getset_def = {
     (char *)"thisown", SwigPyBuiltin_GetterClosure, SwigPyBuiltin_SetterClosure, NULL, &thisown_getset_closure