Add whitespace-before colon ignore

[jsignell]

We do this in other Saturn codebases and it is motivated by PyCQA/pycodestyle#373

The repercussions are we get things like:

-    pet_names_padded = [name[-(str_len + 1):] for name in pet_names_characters]
+    pet_names_padded = [name[-(str_len + 1) :] for name in pet_names_characters]