Django 4.0 compatibility

[Darrel O'Pry]

• https://docs.djangoproject.com/en/4.0/releases/3.1/#id2

  The purely documentational providing_args argument for Signal is deprecated. If you rely on this argument as documentation, you can move the text to a code comment or docstring.

• https://docs.djangoproject.com/en/4.1/releases/4.0/#features-removed-in-4-0

  The providing_args argument for django.dispatch.Signal is removed.

File "...\lib\site-packages\wagtail_headless_preview\signals.py", line 4, in <module>
    preview_update = Signal(providing_args=["token"])
TypeError: Signal.__init__() got an unexpected keyword argument 'providing_args'

[Darrel O'Pry]

Never mind I was on an old release and didn't know it. My bad.