Fix handling of old-style classes

[chrivers]

Pyjaco should either correctly support old-style classes (and we should have unit tests to make sure we have the semantics in place), or (favorably), we should give an error if the user attempts to compile an old-style class.

[dusty-phillips]

I suggest a warning instead of an error.

[chrivers]

Yes, could be. But some kind of notice for sure. I'm not fully sure how different old- and new-style classes are, but I'd like to avoid subtle errors, where possible. So, if we warn the user but we know the code is going to break subtly, that should be an error, I think. But first of all, I have to figure out why it doesn't do anything at the moment.