API : tighten validation on pivot in Quiver

only accept {'mid', 'middle', 'tip', 'tail'} instead of
being super permissive.

Closes #3951
Ah, I am showing my lack of age and have never used a version of python that did not have `{}` for set syntax.   Also was being lazy and assumed that the the 2.6 failure was that speed-up test failing.