Add pre-commit

[florczakraf]

Summary

• we're now using a standard pre-commit tool to perform all the required code style checks
• added isort (it enforces consistent import order)
• updated CI to use pre-commit
• updated dev docs to suggest using pre-commit instead of all the required tools manually
• resolved all issues (mostly missing LF, removed unneeded executable flags, import order -- separate commit with isort)