Start roughing out TEME-to-ITRF conversion
Bring earth-satellite result within 1m with xp, yp
Convert TEME using a theta like Appendix C's
Simplify TEME conversion expressions
Work out the steps equivalent to NOVAS ter2cel()
Cache Earth precession matrix on JulianDate object
Merge branch 'feature/sgp4-integration'

After some experimentation, I now see the way forward to fully
converting SGP4 positions to ICRS positions inside of Skyfield, and so
have tidied up my experimental code and gotten the code base Python 3
compatible again.  So this branch can be merged and further development
can now proceed on master!