test_struct is the only thing that touches __warningregistry__ besides test_warning and (in my other patch) test_py3kwarnings.  Looks like it would benefit from it, and could use some decorators too..

def check_float_coerce(format, number):
  ...
check_float_coerce = with_warning_restore(deprecated_err)

*cough*  Paraphrases of religious quotes relating to the creation of decorators aside, I think that'd be better written as a contextmanager.

By the time I'm done I'm going to end up with two dozen patch sets covering 80% of the python codebase x_x.