Replace deprecated imp module with importlib

[dheater]

Changelog: Fix: The imp module was deprecated with Python 3.4 and is scheduled for removal with Python 3.12. This PR remove the last reference to imp

Closes #12148

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Test with Python 3.6 since that is the oldest version currently supported. Run slow tests to be sure we cover all cases #PYVERS: py36 #TAGS: slow