Fix circular import error from update_backrefs.py

indico/web/forms/fields/principals.py

@@ -14,7 +14,6 @@
 from indico.core.permissions import get_available_permissions, get_permissions_info
 from indico.modules.events import Event
 from indico.modules.events.contributions.models.contributions import Contribution
-from indico.modules.events.registration.util import serialize_registration_form
 from indico.modules.events.roles.util import serialize_event_role
 from indico.modules.events.sessions.models.sessions import Session
 from indico.modules.groups.util import serialize_group
@@ -28,6 +27,7 @@
 
 def serialize_principal(principal):
     from indico.modules.categories.util import serialize_category_role
+    from indico.modules.events.registration.util import serialize_registration_form
     if principal.principal_type == PrincipalType.email:
         return serialize_email_principal(principal)
     elif principal.principal_type == PrincipalType.network:
@@ -177,6 +177,7 @@ def category_roles(self):
 
     @property
     def registration_forms(self):
+        from indico.modules.events.registration.util import serialize_registration_form
         if not self.event.has_feature('registration'):
             return []
         registration_forms = self.event.registration_forms