- moves `pre-commit` to `hooks` group
- uses `isort` to add grouping comments, instead of ignoring them
- `__init__.py` is ignored by `isort` for documentation
- fix missing `-*- coding: utf-8 -*-` annotations
- update `tox` configuration in `pyproject.toml`
- in GitHub actions, `main.yml`:
	- changed CI job name to `PreflightChecks`
	- updated poetry version to `1.4.1`
	- runs `tox` with strategy-matrix
- in `pages.yml`:
	- updated poetry version to `1.4.1`
	- fix typos
- makes flake8 ignore `.venv`