Add missing import in best-practices code example (#25391)
Add missing import in best-practices code example (#25391)

* Add missing import in best-practices code example

This PR adds a missing import to the "unit test for a custom operator" code example. While this code example won't run on its own any way since `MyCustomOperator` isn't defined (and probably shouldn't be for simplicity), I noticed when applying this code to my own custom operator that an import for `DAG` was missing. This PR adds that back in, so the only missing import is for the user-added custom operator.

cfei18/incubator-airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,apache/airflow,nathanielvarona/airflow,apache/airflow,nathanielvarona/airflow,nathanielvarona/airflow,nathanielvarona/airflow,apache/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,apache/airflow,apache/airflow,apache/airflow,cfei18/incubator-airflow