Fix pylint error in tests/

[ashb]

I'm not sure why this changed/started failing, as we haven't touched this part of the file recently.

Maybe we upgraded pylint recently?

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[ashb]

One thing I did notice that #16553 only took 4 seconds for the in https://github.com/apache/airflow/runs/2875380969?check_suite_focus=true

Mon, 21 Jun 2021 13:08:50 GMT Run pylint for main code.................................................Passed
Mon, 21 Jun 2021 13:11:09 GMT Run pylint for tests.....................................................Passed
Mon, 21 Jun 2021 13:11:15 GMT Run pylint for helm chart tests..........................................Passed

4 seconds seems too quite -- running pylint on tests/ locally takes much longer.