[Airflow-4668] Make airflow/contrib/utils Pylint compatible

[swiftomkar]

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Description

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  Makes airflow/contrib/utils pylint compatible

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  No change in code

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Code Quality

• Passes flake8

[ashb]

Changes as they are look good, could you also fix up the associated test files:

./tests/contrib/utils/logging_command_executor.py
./tests/contrib/utils/run_once_decorator.py
./tests/contrib/utils/test_sendgrid.py
./tests/contrib/utils/test_weekday.py

[ashb]

While we are here making changes could you correct these to:

         self.assertIsIinstance(e, WeekDay)

etc

[swiftomkar]

@ashb Done!