Renovations and require Python >=3.7

[dbast]

Only renovations and disabling Python 2.7/3.6 support (no functional code changes)

• Switch from flake8 only to pre-commit (including flake8 and more checks): Found and fixed a broken shebang.
• Apply and check import orders with isort
• remove Python 2.7+3.6 support due to EOL (tests already run with Python 3.7 as lower and 3.10 as upper bound)
• Upgrade code to py37-plus with pyupgrade: Python2 conditionals get removed and string handling is consistently converted to f-string syntax.

This is all done via atomic commits, where each one runs green on CI (please merge without squash).