scipy ridder fails

```
648     if rtol < _rtol:
    649         raise ValueError("rtol too small (%g < %g)" % (rtol, _rtol))
--> 650     r = _zeros._ridder(f, a, b, xtol, rtol, maxiter, args, full_output, disp)
    651     return results_c(full_output, r)
```