In Python 2.7, the inspect.isclass() method became more restrictive,
and the python documentation system stopped recognizing VTK classes
as true Python classes, since VTK classes still use a mechanism from
the days of Python 1.5.  This patch makes PyVTKClass into a subclass
of PyClass, and now the inspect module recognizes VTK classes as true
Python classes.

Change-Id: I83fa6b5adb7fdef53036069a83a2dae64dffd4ae