Currently types.SimpleNamespace does not support weak references.  Amaury asked for this in #14673, but I'd like to see more discussion.  What are the use cases for weak references in this case?

The type is a simple wrapper around dict, which does not support weak references.  In fact most of the builtin types do not support them.  However, I freely admit that I haven't used weak references very much and am not familiar with the use cases.