DX: Work-around: Tox crashes on flake8 format setting.

- Running flake8 via tox, e.g., `tox -e flake8` crashes.
  Setuptools uses configparser to read the config, which
  throws configparser.InterpolationMissingOptionError because
  of the percent (%) symbols in the format string.
  - Ref:
    pytest-dev/pytest#3062
    pypa/pip#5182

.flake8

@@ -0,0 +1,5 @@
+# vim:tw=0:ts=2:sw=2:et:norl:ft=config
+[flake8]
+# Add a space after the path to make the output path easier to double-click-copy.
+format=%(path)s %(row)d:%(col)d: %(code)s %(text)s
+

MANIFEST.in

@@ -36,6 +36,7 @@ graft tests
 # - NOTE: Order this like `git ls-files`, to keep it easy to maintain.
 exclude .coveragerc
 exclude .editorconfig
+exclude .flake8
 exclude .gitignore
 exclude .ignore.example
 exclude .readthedocs.yml

setup.cfg

@@ -128,8 +128,6 @@ exclude =
     .pytest_cache/
     __pycache__
 max-line-length = 89
-# Add a space after the path to make the output path easier to double-click-copy.
-format=%%(path)s %%(row)d:%%(col)d: %%(code)s %%(text)s
 # See list of error codes (we can list here to ignore):
 #
 #   https://pycodestyle.readthedocs.io/en/latest/intro.html#error-codes