Python tweaks

A couple of tweaks to consider.

The super snippet I suppose relies on people using python.el, so having python-info-current-defun available.  Not sure if that's acceptable
I really can't say about various versions of `python-info-current-defun`...  I'm just using what comes with my Emacs 24.3.1 (python-mode 0.24.2).  I don't have handy access to older versions.

I can say that it works fine for nested classes, special function like `__init__` etc..