ContextTempOverride objects were not freeing the reference to their
arguments when the context managers __exit__ didn't run.
This would happen if __enter__ failed with an unsupported configuration
or if object was created and not used.
Resolve by making ContextTempOverride a container type with GC support.
This also allows the ContextTempOverride to be reused since it's
arguments are now cleared when it's deallocated instead of __exit__.
Campbell Barton
9363ad59b0