Phase 1: Can we make c wrappers immutable by default?

[xFrednet]

The Py_MakeImmutable implementation has some special handling for c wrapper types with a comment:

_Py_SetImmutable(item);

if(is_c_wrapper(item)) {
    // C functions are not mutable, so we can skip them.
    goto next;
}

After talking to @mjp41 it sounds like C functions are immutable, but doing a deep freeze would cause a lot of problems, which is why Py_MakeImmutable just processes the next item. However, my add_to_region and try_close_region implementation run into the same objects and also need to handle the c wrapper types. Currently, I've opted to just freeze them, but that adds another case, which needs a lot of explanation.

So my question is: Could we just spawn c wrapper types as immutable by default?

cc: @matajoh