Get rid of astropy.erfa deprecation warning

[mattpitkin]

If using astropy v4.2 or higher you get a deprecation warning about importing the astropy._erfa library as erfa is now a separate package (pyerfa), which is a requirement of astropy.

This PR bumps the required version of astropy for libstempo to be at least 4.2 and also imports erfa directly, thus getting rid of the deprecation warning.