Add a code formatting tool chain

- (auto-)format code with:
  + autoflake => * remove unused imports and variables
                 * remove duplicate dict keys
                 * expand star imports
  + black => enforce an uncompromising code style
  + isort => enforce a consistent import style
             (complying with Google's Python Style Guide)
- implement the nox session "format" that runs all these tools
- add the following file:
  + setup.cfg => holds configurations for the develop tools