Access GILProtected via PyVisit

[neachdainn]

Since you can't access a Python<'_> from within __traverse__, it becomes impossible to access a GILProtected object while traversing, which causes problems if there is an object that needs to be traversed somewhere inside the GILProtected. Seeing as GILProtected::get doesn't actually do anything, this should be a valid operation despite the restrictions imposed during GC.

Rather than rehashing the discussions from previous issues regarding marker access during GC, it seems relatively straight-forward to simply allow access to the inner object using a PyVisit object.

[neachdainn]

From previous discussions:

I believe you can still borrow PyCell from traverse. We can't use the GIL, but we can still make use of the fact that there is at most one thread with the GIL.

The problem is that PyCell derefs into PyAny which provides implicit access to a GIL token via PyAny::py, i.e. we cannot even expose &PyCell<Self> or any &PyAny to the traverse implementation if it is not supposed to safely assume holding the GIL.

Accessing the GILProtected object can make use of the fact that there is at most one thread with the GIL while also not providing any new access to a GIL token.