`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However, DataArray.interp still has an outdated check for numeric dtypes, meaning that other dtypes (e.g. datetimes) result in a TypeError: https://github.com/pydata/xarray/blob/f1a2c08f21a8237c082913fcad2d77469448cf4c/xarray/core/dataarray.py#L2357