Signal has historically contained a providing_args= argument, which

listed the names of the arguments expected to be sent to the signal.

This was optional and never used for anything beyond the ability to

introspect a signal (which probably nobody does), and isn't supported on

modern versions of Django.
This change removes providing_args= from all signals.

Unit tests pass on Django 1.11 and on 3.2 (with other in-progress changes).