BUG: interpolate: sproots works only for cubic splines

The `sproot` subroutine in FITPACK actually [works correctly only for cubic splines](https://github.com/scipy/scipy/blob/master/scipy/interpolate/fitpack/sproot.f#L2).

The `sproot` function in Scipy attempted to do something also with splines of other orders, but there's no mathematical justification for what was attempted, and it produces wrong results. So, accept only order 3 splines from now on. (UnivariateSpline.roots correctly accepts only order 3 splines.)

Fixes gh-2438