Set flake8 line length

Although we don't currently use flake8 in CI, this is nice for anyone
who has it enabled in their editor.

Author: oschwald

setup.cfg

@@ -2,5 +2,9 @@
 build_html = build_sphinx -b html --build-dir docs
 sdist = build_html sdist
 
+[flake8]
+# black uses 88 : ¯\_(ツ)_/¯
+max-line-length = 88
+
 [wheel]
 universal = 1