Move flake8's config to its own file

.flake8

@@ -0,0 +1,11 @@
+# TODO: incorporate this into pyproject.toml if flake8 supports it in the future.
+# See https://github.com/PyCQA/flake8/issues/234
+[flake8]
+# see https://pycodestyle.readthedocs.io/en/latest/intro.html#error-codes
+# for error codes. The ones we ignore are:
+#  W503: line break before binary operator
+#  W504: line break after binary operator
+#  E203: whitespace before ':' (which is contrary to pep8?)
+#  E731: do not assign a lambda expression, use a def
+#  E501: Line too long (black enforces this for us)
+ignore=W503,W504,E203,E731,E501

setup.cfg

@@ -7,15 +7,6 @@ ignore =
     pylint.cfg
     tox.ini
 
-[flake8]
-# see https://pycodestyle.readthedocs.io/en/latest/intro.html#error-codes
-# for error codes. The ones we ignore are:
-#  W503: line break before binary operator
-#  W504: line break after binary operator
-#  E203: whitespace before ':' (which is contrary to pep8?)
-#  E731: do not assign a lambda expression, use a def
-#  E501: Line too long (black enforces this for us)
-ignore=W503,W504,E203,E731,E501
 
 [isort]
 line_length = 88