In Python 3.6 a number of escape sequences that were previously accepted
-- for instance "\(" that was translated to "\\(" -- are deprecated. To
retain the previous behavior either raw strings must be used or the
backslash must be properly escaped itself.

The strings in error messages were left untouched

Simplify the logic a bit and make behavior consistent with polyval.

Note that this adds a check that the number of dimensions of the
array of the evaluation points is less than the number of dimensions of
the array of roots when tensor=False. That check is missing from polyval
and needs to be added.

As one can easily encounter when working with high-order signal processing
filters, converting a high-order polynomial from its roots to its polynomial
coefficients can be quite lossy, leading to inaccuracies in the filter's
properties.

This PR adds a new function, `polyrootval` - based on `polyval` -  that
evaluates a polynomial given a list of its roots. The benefit of calculating it
this way can be seen at scipy/scipy:6059. Some tests are included, as well.

* Rewrite the documentation for `deg`.
* Check that the passed degrees are int and raise if not.
* Do not accept `deg` arguments that do not convert as 0-D or 1-D.
* Sort passed degree terms for repeatability.