BUG: core.missing._akima_interpolate will raise AttributeError
```
    from scipy import interpolate

    P = interpolate.Akima1DInterpolator(xi, yi, axis=axis)

    if der == 0:
        return P(x)
    elif interpolate._isscalar(der):
        return P(x, der=der)
    else:
        return [P(x, nu) for nu in der]
```

There is no `interpolate._isscalar` so this would raise if it were ever reached.  AFAICT we have only one test that gets to this function, must always have `der == 0`