scripting: fix Python initialization

Apparently with Python 3.9 Py_Initialize() is needed even before
creating PyObject's, i.e. what the default constructor of
boost::python:dict does by calling PyDict_New().

Hence, move the insertion of the 'kig' module followed by
Py_Initialize() in the constructor of a separate helper class: this
class will be a private base for PythonScripter::Private, which thus can
properly initialize boost::python objects.