Signup: use the shorthand version of setting an unusable password.

It wasn't actually saving the unusable password before.

django_etebase/serializers.py

@@ -397,13 +397,11 @@ class AuthenticationSignupSerializer(serializers.Serializer):
                 instance = User.objects.get_by_natural_key(user_data['username'])
             except User.DoesNotExist:
                 # Create the user and save the casing the user chose as the first name
-                instance = User.objects.create_user(**user_data, first_name=user_data['username'])
+                instance = User.objects.create_user(**user_data, password=None, first_name=user_data['username'])
 
             if hasattr(instance, 'userinfo'):
                 raise serializers.ValidationError('User already exists')
 
-            instance.set_unusable_password()
-
             try:
                 instance.clean_fields()
             except django_exceptions.ValidationError as e: