Linters are run multiple times on CI

[akaihola]

In the build matrix, multiple different OS / Python version configurations run the same linters:

• on Linux/Win/macOS with Python 3.7 thru 3.10:
  • flake8 via pytest-flake8
  • isort via pytest-isort

mypy and pylint are run as part of the darker run on Linux/Win/macOS with Python 3.9 and 3.10. But we could argue that's useful duplication since it exercises the --lint functionality of Darker on different platforms.