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

setup.cfg

@@ -0,0 +1,26 @@
+[black]
+# black's settings are in pyproject.toml => [tool.black]
+
+
+[isort]
+atomic = true
+case_sensitive = true
+combine_star = true
+force_alphabetical_sort_within_sections = true
+lines_after_imports = 2
+remove_redundant_aliases = true
+
+# Comply with black's style.
+# Source: https://github.com/psf/black/blob/master/docs/compatible_configs.md#isort
+ensure_newline_before_comments = true
+force_grid_wrap = 0
+include_trailing_comma = true
+line_length = 88
+multi_line_output = 3
+use_parentheses = true
+
+# Comply with Google's Python Style Guide.
+# All imports go on a single line except the ones from the typing module.
+# Source: https://google.github.io/styleguide/pyguide.html#313-imports-formatting
+force_single_line = true
+single_line_exclusions = typing