Fix issue #596 'slice is not subclassable'

[corona10]

Fix issue #596
make slice to be not subclassable.

[undingen]

LGTM, thanks for the patch!

While this fixes it for the slice type there are a bunch of other types which still allow subclassing even they should not. I'm wondering if we should not at some point switch to a approach where types have to explicitly say what additional flags they want to have because it may be less work and more similar to the capi and cpython.

[corona10]

@undingen Thanks i agree with your opinion. Do you know any good document or article to get the relevant information about which not to be subclassable types. When i know that lists than i can try to find another approach.

[undingen]

I don't know of a list of types which don't allow subclassing :-(.
My approach would be to remove the line inside the constructor of BoxedClass which always enables subclassing. And than add the missing flags to all our C++ types which show up inside a search for Py_TPFLAGS_BASETYPE inside cpythons Objects/* dir. CAPI types should all be fine so I suspect this are not many types which need it and this are probably mostly types where we already are modifying the tp_flags member.