Avoid scipy deprecation warning on RectBivariateSpline import

[hassec]

Small change to import RectBivariateSpline from scipy.interpolate instead from deprecated fitpack2 submodule.

The requirements.txt doesn't really specify what the minimum version of scipy is that omas supports. But I noticed that there is already a

from scipy.interpolate import RectBivariateSpline

further down in the same file, thus this change is fully backwards compatible and avoids the import warning when using newer scipy versions.