scipy.interpolate.interp2d is deprecated

The GWpy Q-transform code uses scipy.interpolate.interp2d which is now deprecated, and emits this warning:

/home/duncan.macleod/gwpy-nightly-build/gwpy/gwpy/signal/qtransform.py:568: DeprecationWarning: `interp2d` is deprecated!
`interp2d` is deprecated in SciPy 1.10 and will be removed in SciPy 1.12.0.

For legacy code, nearly bug-for-bug compatible replacements are
`RectBivariateSpline` on regular grids, and `bisplrep`/`bisplev` for
scattered 2D data.

In new code, for regular grids use `RegularGridInterpolator` instead.
For scattered data, prefer `LinearNDInterpolator` or
`CloughTocher2DInterpolator`.

For more details see
`https://gist.github.com/ev-br/8544371b40f414b7eaf3fe6217209bff`

  interp = interp2d(xout, frequencies, out.value.T, kind='cubic')