[capi-sig] Instantiation of subclasses of ctypes

The Structure interface of ctypes is intended to be subclassed yet the subclasses init and new aren't invoked. I think this was intended as an optimization but it is inconsistent and confusing. The only documentation about this behavior takes the form of a unit test, which may not have ever worked properly.

There is an issue about this open here https://bugs.python.org/issue38860

I'm of the opinion it should not be "special", what does the community think?