Future: `_traverse_or_findall` -> `findall` when docutils 0.18 is the minimum version

[rossbar]

Just opening a tracking issue for future cleanup. The _traverse_or_findall shim can be removed in the future when the minimum supported docutils is bumped to 0.18. See #403 for details.

This is noted comments in code, but I thought I'd add it to the issue tracker as well for increased visibility.

[jarrodmillman]

We just need to remember to close this when we update the code.