Fix RemovedInDjango40Warning from Signal arguments

Fix to removove the following warning I've started to see when running unittest in my django project.

"""/<path to venv/lib/python3.7/site-packages/django_registration/signals.py:13: RemovedInDjango40Warning:
The providing_args argument is deprecated. As it is purely documentational, it has no replacement.
If you rely on this argument as documentation, you can move the text to a code comment or docstring.

user_activated = Signal(providing_args=["user", "request"])"""

Author: TomasLoow

src/django_registration/signals.py

@@ -7,7 +7,9 @@
 
 
 # A new user has registered.
-user_registered = Signal(providing_args=["user", "request"])
+# Provided args: user, request
+user_registered = Signal()
 
 # A user has activated his or her account.
-user_activated = Signal(providing_args=["user", "request"])
+# Provided args: user, request
+user_activated = Signal()