Fix flake8 configuration

Apparently flake8 v3 doesn't like trailing comments on config settings.

Also remove the pep8 config, which didn't work (because it was missing W503)
and duplicated the flake8 config. We don't use pep8 on its own, so the config
was duplicative.

Author: richvdh

setup.cfg

@@ -16,7 +16,5 @@ ignore =
 
 [flake8]
 max-line-length = 90
-ignore = W503 ; W503 requires that binary operators be at the end, not start, of lines. Erik doesn't like it.
-
-[pep8]
-max-line-length = 90
+#  W503 requires that binary operators be at the end, not start, of lines. Erik doesn't like it.
+ignore = W503