Add async support to PermissionRequiredMixin

[g-nie]

The end goal is to be able to do:

from asgiref.sync import sync_to_async


class MyView(PermissionRequiredMixin, View):
    permission_required = 'some_perm'
    raise_exception = True

    async def get_permission_object(self):
        return await MyModel.objects.aget(slug='awesome-slug')

    async def get(self, request, *args, **kwargs):
        return await sync_to_async(render_to_string)('some_template.html')

Django >= 4.1.2 is needed (as indicated by the skipIf's in tests as well), since that seems to be a stable point of async views in Django. For reference, a bugfix related to non-allowed HTTP methods was fixed in 4.1.2 (release notes, ticket)