changed caching behavior

[markusmo]

I just setup my shop together with django-redis-cache package and I got an error, when changing a model, when it wanted to invalidate mny
So I adapted the code in product.py to use Django caching API's delete(key).

See: https://docs.djangoproject.com/en/3.0/topics/cache/#django.core.caches.cache.delete

[jrief]

OK. This looks like an API change in the redis-cache library. Can we restrict the versions which are compatible with those changes.

[markusmo]

Well redis-cache did not work with the current new version, so I had to upgrade. Using the standard django caching will result in redis-cache to be triggerd or any other cache that will be used. So this would be an agnostic aproach