Add flake8 configuration file

After blackening the repo I saw that the Travis build was failing. I saw that
it was due to incompatibility between default the default flake8 configuration
and the newly blackened code. So I added the .flake8 configuration file we use
in skeleton-python-library as an interim to fully switching this project over
to GitHub Actions from Travis.

Author: mcdonnnj

.flake8

@@ -0,0 +1,25 @@
+[flake8]
+max-line-length = 80
+# Select (turn on)
+# * Complexity violations reported by mccabe (C) -
+#   http://flake8.pycqa.org/en/latest/user/error-codes.html#error-violation-codes
+# * Documentation conventions compliance reported by pydocstyle (D) -
+#   http://www.pydocstyle.org/en/stable/error_codes.html
+# * Default errors and warnings reported by pycodestyle (E and W) -
+#   https://pycodestyle.readthedocs.io/en/latest/intro.html#error-codes
+# * Default errors reported by pyflakes (F) -
+#   http://flake8.pycqa.org/en/latest/glossary.html#term-pyflakes
+# * Default warnings reported by flake8-bugbear (B) -
+#   https://github.com/PyCQA/flake8-bugbear#list-of-warnings
+# * The B950 flake8-bugbear opinionated warning -
+#   https://github.com/PyCQA/flake8-bugbear#opinionated-warnings
+select = C,D,E,F,W,B,B950
+# Ignore flake8's default warning about maximum line length, which has
+# a hard stop at the configured value.  Instead we use
+# flake8-bugbear's B950, which allows up to 10% overage.
+#
+# Also ignore flake8's warning about line breaks before binary
+# operators.  It no longer agrees with PEP8.  See, for example, here:
+# https://github.com/ambv/black/issues/21. Guido agrees here:
+# https://github.com/python/peps/commit/c59c4376ad233a62ca4b3a6060c81368bd21e85b.
+ignore = E501,W503