Comment 4 for bug 1834614

At disco the patches to default to pymysql for the django mysql driver where dropped; so the app is looking for mysqldb rather than pymysql.

This is not configurable - the only way to force pymysql is this hack:

  https://stackoverflow.com/questions/34777755/how-to-config-django-using-pymysql-as-driver

so we might just be better to install python3-mysqldb to fix this issue. Horizon is not eventlet dependent so some of the benefits of using pymysql are not so valuable.