vtkCollection now supports Python sequence protocol

All vtkCollection subclasses now support len(), indexing with [] (including negative indices), and membership testing with in:

collection = vtkActorCollection()
collection.AddItem(actor1)
collection.AddItem(actor2)

len(collection)        # 2
collection[0]          # actor1
collection[-1]         # actor2
actor1 in collection   # True

This is implemented at the C level via tp_as_sequence, so it is automatically inherited by all ~23 subclasses of vtkCollection (e.g., vtkRendererCollection, vtkDataArrayCollection, etc.) without any additional Python code.

Pythonic mutation methods are also available:

collection.append(actor3)     # AddItem
collection.insert(0, actor4)  # InsertItem
collection.remove(actor1)     # RemoveItem
collection.clear()            # RemoveAllItems

All of this is implemented at the C level and automatically inherited by all ~23 subclasses of vtkCollection (e.g., vtkRendererCollection, vtkDataArrayCollection, etc.) without any additional Python code.

The existing iteration protocol (for item in collection) continues to work as before.