pyloxi: create inheritance hierarchy

Virtual classes are generated but currently only serve as superclasses.

Somehow this works without topologically sorting the classes.
diff --git a/py_gen/templates/_virtual_ofclass.py b/py_gen/templates/_virtual_ofclass.py
new file mode 100644
index 0000000..29f01f7
--- /dev/null
+++ b/py_gen/templates/_virtual_ofclass.py
@@ -0,0 +1,3 @@
+:: superclass_pyname = ofclass.superclass.pyname if ofclass.superclass else "loxi.OFObject"
+class ${ofclass.pyname}(${superclass_pyname}):
+    pass