Apache Airflow version: v1.10.11

Kubernetes version (if you are using kubernetes) (use kubectl version): NO

Environment: airflow 1.10.11 running on ubuntu 18.04 (used pip for installation)

• Cloud provider or hardware configuration: bare metal
• OS (e.g. from /etc/os-release): ubuntu 18.04
• Kernel (e.g. uname -a): Linux dev1 4.15.0-108-generic 109-Ubuntu SMP Fri Jun 19 11:33:10 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
• Install tools: pip
• Others:

What happened:
I'm not able to get task_instance_mutation_hook (#8852) to load my custom implementation from $AIRFLOW_HOME/config/airflow_local_settings.py.

What you expected to happen: Airflow to use task_instance_mutation_hook provided in $AIRFLOW_HOME/config/airflow_local_settings.py.

How to reproduce it: make sure your running v1.10.11 and create a custom $AIRFLOW_HOME/config/airflow_local_settings.py where you define a dummy task_instance_mutation_hook as next

def task_instance_mutation_hook(ti):
   # every new or up to retry task will be added to non existent queue, so no task will pass through
   if ti.state in [State.NONE, State.UP_FOR_RETRY]:
      ti.queue = 'dummy-queue'

if any of the airflow example dags run successfully then, the previous hook didn't work.

Anything else we need to know: task_instance_mutation_hook is a new feature introduced in v1.10.11