Deprecations for 3.0

[Andrew Whitworth]

> - there are places (not in PMCs) that manually change PMC
> attributes, not through vtable calls.

Keep in mind that we are talking about a major redesign of our object
metamodel in the coming months as well, so the current list of vtables
may be completely replaced. If we need a vtable to handle attribute
access in a standard way, we will probably have the opportunity to
make that happen.

--Andrew Whitworth