flake8: we want line breaks before binary operators

This is recommended by PEP 8.

Also remove some warnings that are no longer raised.

Reference: https://www.python.org/dev/peps/pep-0008/#should-a-line-break-before-or-after-a-binary-operator

setup.cfg

@@ -4,6 +4,8 @@ ignore =
  E122,
  # line too long
  E501,
+ # project choice; PEP 8 recommends line breaks before binary operators:
+ W503,
  # Ignore all flake8 errors for now. Whoever wants to fixup our code (PLEASE),
  # remove one, fixup all occurences, submit a merge request...
  E123,
@@ -26,6 +28,3 @@ ignore =
  F812,
  F821,
  F841,
- W291,
- W293,
- W503