pwkit/astutil.py: whoops, previous change added a hard dependency on skyfield
pwkit/astutil.py: whoops, previous change added a hard dependency on skyfield

I'd like the bulk of this module's functionality to still work if Skyfield
isn't available, so add the necessary logic to import successfully if that's
the case.

(Note that Python 3.6 has introduced ModuleNotFoundError, but it's a subclass
of ImportError, so we can use the latter and retain 2/3 portability.)

pkgw/pwkit