txaws has some code that constructs Attribute directly (it is public, after all). It does so by passing a value for the default parameter by keyword (it is public, after all).

https://github.com/twisted/txaws/blob/8127b45978aa7b91e65eae15546a63440932b57f/txaws/client/_validators.py#L46-L54

This worked prior to 17.1.0 but no longer does.