admin: use decorator syntax

This makes the code easier to read.

social_django/admin.py

@@ -8,6 +8,7 @@
 from .models import Association, Nonce, UserSocialAuth
 
 
+@admin.register(UserSocialAuth)
 class UserSocialAuthOption(admin.ModelAdmin):
     """Social Auth user options"""
 
@@ -48,21 +49,18 @@ def _get_all_field_names(model):
         return list(set(names))
 
 
+@admin.register(Nonce)
 class NonceOption(admin.ModelAdmin):
     """Nonce options"""
 
     list_display = ("id", "server_url", "timestamp", "salt")
     search_fields = ("server_url",)
 
 
+@admin.register(Association)
 class AssociationOption(admin.ModelAdmin):
     """Association options"""
 
     list_display = ("id", "server_url", "assoc_type")
     list_filter = ("assoc_type",)
     search_fields = ("server_url",)
-
-
-admin.site.register(UserSocialAuth, UserSocialAuthOption)
-admin.site.register(Nonce, NonceOption)
-admin.site.register(Association, AssociationOption)