Use black code formatter and enable it in a pre-commit hook

[nuest]

• https://github.com/psf/black#version-control-integration
• read https://ljvmiranda921.github.io/notebook/2018/06/21/precommits-using-black-and-flake8/
• Decide if to use GitHub actions, Travis, or both
• Added, configured and documented for geoextent

[nuest]

GitHub Actions

1. flake8 snippet removed from template

   - name: Lint with flake8
     run: |
       pip install flake8
       # stop the build if there are Python syntax errors or undefined names
       flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
       # exit-zero treats all errors as warnings. The GitHub editor is 127 chars wide
       flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics

2. black action: https://github.com/marketplace/actions/black-code-formatter