bugfix: nested directives were broken in python 3

- The nested directive implementation was broken for python 3

- directive results were not properly removed from the directive list
  when it was processed in the DirectiveMeta metaclass.

- the issue was that remove_directives only descended into a list or
  tuple, but in Python3, the initial value passed to the function is a
  view of dictionary values.

- make it a list to fix things, and add a regression test.