fix require_invitation option, with this active it was not possible t…

…o vote

bitpoll/poll/models.py

@@ -88,7 +88,7 @@ def can_vote(self, user: AbstractUser, request: Optional[HttpRequest] = None, is
             if request:
                 messages.error(request, _("Login required to vote."))
             return False
-        elif self.require_invitation and (not user.is_authenticated or user not in self.invitation_set.all().values('user')):
+        elif self.require_invitation and (not user.is_authenticated or not self.invitation_set.filter(user=user).exists()):
             if request:
                 messages.error(request, _("You are not allowed to vote in this poll. You have to be invited"))
             return False