`DataArray.interp` only works for numeric dtypes

[w-k-jones]

What is your issue?

Dataset.interp was updated to allow non-numeric dtypes in #5008

However, DataArray.interp still has an outdated check for numeric dtypes, meaning that other dtypes (e.g. datetimes) result in a TypeError:

xarray/xarray/core/dataarray.py

Line 2357 in f1a2c08

if self.dtype.kind not in "uifc":