Added a missing import for resources in the mixins module.
Added a missing import for resources in the mixins module.

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