C/C++ extension: subclassing an extension type with Python code?

[Regular Expression]

I'd like to create an extension type which can be further extended by end
users of the type. Ideally, I'd like to define certain properties that are
read-only, and others that cannot be occluded in "subclasses" of the type.
I'd also like to define certain methods that cannot be overridden.

Does anybody have an idea how to do this?