Defined self.request in dispatch before calling a view method that depen...

`GroupRequiredMixin.check_membership()` accesses `self.request.user.groups.values_list()`. This causes `GroupRequiredMixin` to throw an `AttributeError` when used in a view as `self.request` is not yet defined until the `super` call at the end of `dispatch`.

This leads me to wonder if I'm the first person really using `GroupRequiredMixin`... :+1: