API : tighten validation on pivot in Quiver

only accept {'mid', 'middle', 'tip', 'tail'} instead of
being super permissive.

Closes #3951
I know, but the set syntax is why it is failing on Python 2.6.  A set confers no advantages over a tuple or list in this use case.  The `{}` set literal syntax was backported from 3.1 to 2.7 only.