Isolate viewset get permission from model

[sergioisidoro]

Based on #155 , attempting to isolate the dependency of viewsets from the Model Mixins.

This allows users to avoid the RulesModelMixin by defining a custom viewset mixin:

from rules.contrib.rest_framework import AutoPermissionViewSetMixin

MyCustomAutoPermissionViewSetMixin:
  def get_permission_for_model(self, model, perm_type):
       return "%s.%s_%s" % (model._meta.app_label, perm_type, model._meta.model_name)

....

class PostViewSet(MyCustomAutoPermissionViewSetMixin, ModelViewSet):
    queryset = Post.objects.all()
    serializer_class = PostSerializer

[sergioisidoro]

Tests failing because of Django 4.0 -> #158

[sergioisidoro]

Rebased this and #157 to master, with the test fixes.