Closes#3615
This simply adds a new method which uses the existence of a `PyVisit`
object as proof that the GIL is held instead of a `Python` object. This
allows `GILProtected` to be used in instances where contained Python
objects need to participate in garbage collection. Usage in this
situation should be valid since no Python calls are made and this does
not provide any additional mechanism for accessing a `Python` object.
