Declare mixins used by resource on class declaration.
Declare mixins used by resource on class declaration.

Previously they were dynamically added to the base class, which makes the
code harder to read.

SmileyJames/shopify_python_api,metric-collective/shopify_python_api,ifnull/shopify_python_api,gavinballard/shopify_python_api,varesa/shopify_python_api,asiviero/shopify_python_api,Shopify/shopify_python_api