Use six from packages

[hmstepanek]

Overview

Replace six imports with package six. Fix some lint issues at the same time.

[github-actions[bot]]

🦙 MegaLinter status: ❌ ERROR

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ PYTHON	black	17	0	0	1.33s
✅ PYTHON	flake8	17		0	0.62s
✅ PYTHON	isort	17	0	0	0.22s
❌ PYTHON	pylint	17		68	7.43s

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