Update Gpib.py to use new-style class

This changes the object-oriented version of the python binding (Gpib.py) to use a new-style class rather than an old-style class. This is a useful enhancement for subclassing Gpib in user code - e.g., modifying the read() method to strip termination characters using python's super() builtin.

New-style classes were introduced in python 2.2 - so they're not so new anymore.