Fixing SlackAPIFileOperator

[BorysWix]

Details #14880

[boring-cyborg]

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[github-actions]

The Workflow run is cancelling this PR. It has some failed jobs matching ^Pylint$,^Static checks,^Build docs$,^Spell check docs$,^Provider packages,^Checks: Helm tests$,^Test OpenAPI*.

[mik-laj]

Can you add tests to avoid regression?

[michaelosthege]

The code from this PR didn't work for me (using the SlackHook directly from PythonOperator).
Maybe I did something wrong, but what works for me (Airflow 2.0.1) is the code from this: https://stackoverflow.com/a/58264852/4473230

[eladkal]

Borys won't be able to complete this PR