Add compatibility for Python 3.6: follow-up

This is a follow-up on !132 (merged) which fixes some stuff that I missed.

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• 902fa1a7 - Compatibility with str.removeprefix

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• 79e4743f - Compatibility with str.removeprefix

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approved this merge request

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• 14b89b24 - Fix typo: stdin -> stderr

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merged

mentioned in commit 44c1c68a