python: Fix the build for Python >= 3.

python: Fix the build for Python >= 3.

SWIG_init() returns void for Python 2.x and 'PyObject *' for Python 3.

Use an #if to handle both cases properly, otherwise the Python bindings
for either Python 2 or 3 will fail to build.

Python 3.x failure:

sigrok/core/classes_wrap.cpp: In function ‘PyObject* PyInit__classes()’:
sigrok/core/classes_wrap.cpp:59002:5: error: return-statement with no
value, in function returning ‘PyObject* {aka _object*}’ [-fpermissive]
     return;
     ^

Python 2.x failure:

In file included from /usr/include/dirent.h:244:0,
                 from /usr/include/glib-2.0/glib/gdir.h:32,
                 from /usr/include/glib-2.0/glib.h:45,
                 from /usr/include/pygobject-3.0/pygobject.h:7,
                 from sigrok/core/classes_wrap.cpp:3179:
sigrok/core/classes_wrap.cpp: In function ‘void init_classes()’:
sigrok/core/classes_wrap.cpp:59002:12: error: return-statement with a
value, in function returning 'void' [-fpermissive]
     return NULL;
            ^