Replace PyEphem with astropy or skyfield

[dkirkby]

The pyephem library is now deprecated. This issue is to replace it with either astropy or skyfield, preferring astropy unless it is missing some functionality or too slow.

The use of pyephem is an implementation detail and will not change any public API or the format of the current ephemerides file, so this is not an urgent task (but still worth doing given the DESI timescale).