TRAINING ISSUE! Add extra CC: to the emails sent by Airflow

[ashb]

Description

We need to have a way to add extra CC: to the emails sent by Airflow

Use case / motivation

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This is a test issue that is used in Contributor's documentation to show an example issue for the first time contributors

Please do not merge the change even if you implement it!

The example workflow that uses this issue is described in
https://github.com/apache/airflow/blob/master/CONTRIBUTING.rst#contribution-workflow-example

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We need to have a way to add extra CC: to the emails sent by Airflow.

1. You can see the information about how to configure email here email: [https://airflow.readthedocs.io/en/latest/concepts.html#email-configuration]
2. Here is the class you should modify: airflow/utils/email.py
3. Tests for email class are in: tests/utils/test_email.py
4. Run the tests in Local Virtualenv: LOCAL_VIRTUALENV.rst
5. Run the tests in Breeze: BREEZE.rst
6. Make a DRAFT PR as described in https://github.com/apache/airflow/blob/master/CONTRIBUTING.rst#contributions
7. Run airflow webserver and scheduler and configure it to send emails and connect to a real SMTP server and test if the email is sent with the CC:'d account