Make __dict__ accessible for Python builtin classes

Attribute set within instance of a SWIG Python wrapped class are
stored in SwigPyObject->dict, which tp_dictoffset slot is pointing to.

However, SWIG wrapped classes did not have a __dict__ attribute.
Inheriting subclasses did not get the attribute either because the
SWIG wrapped classes initialize the tp_dictoffset slot:

From http://bugs.python.org/issue16272:

"If a type defines a nonzero tp_dictoffset, that type is responsible for
defining a `__dict__` slot as part of the tp_getset structures. Failure to
do so will result in the dict being inaccessible from Python via
`obj.__dict__` from instances of the type or subtypes."

Provide a SwigPyObject_get___dict__() function to retrieve the dict
attribute or create it when it does not exist yet (it is normally
created when setting attribute set), and a PyGetSetDef entry pointing
to this function.

Lib/python/pyrun.swg

@@ -381,6 +381,23 @@ typedef struct {
 #endif
 } SwigPyObject;
 
+
+#ifdef SWIGPYTHON_BUILTIN
+
+SWIGRUNTIME PyObject *
+SwigPyObject_get___dict__(PyObject *v, PyObject *SWIGUNUSEDPARM(args))
+{
+  SwigPyObject *sobj = (SwigPyObject *)v;
+
+  if (!sobj->dict)
+    sobj->dict = PyDict_New();
+
+  Py_INCREF(sobj->dict);
+  return sobj->dict;
+}
+
+#endif
+
 SWIGRUNTIME PyObject *
 SwigPyObject_long(SwigPyObject *v)
 {