BUG: Set __hash__ = None for non-hashable classes.

Because neither poly1d nor the Polynomial package polynomial classes are
immutable, hence not reliably hashable, they should signal that by
setting __hash__ = None. This also fixes the warning

Overriding __eq__ blocks inheritance of __hash__ in 3.x

that is given when the command `python2.7 -3 -c"import numpy"` is run.
Merge pull request #3684 from charris/backport-to-1.8-3657

Backport to 1.8 3657