GPL dependency error installing Airflow 1.10

When installing Airflow 1.10 with pip, I received the following error:

RuntimeError: By default one of Airflow's dependencies installs a GPL
dependency (unidecode). To avoid this dependency set
SLUGIFY_USES_TEXT_UNIDECODE=yes in your environment when you install or upgrade
Airflow. To force installing the GPL version set AIRFLOW_GPL_UNIDECODE

As the error message explains, the solution is to set the environment variable when calling pip:

SLUGIFY_USES_TEXT_UNIDECODE=yes pip install apache-airflow==1.10.0