pyfits are obsolete

Pyfits is obsolete and will not get any upstream updates anymore. It is
replaced by Astropy which provides a drop-in-replacement as astropy.io.fits.
Please have a look on this  [patch](https://anonscm.debian.org/cgit/debian-med/pyscanfcs.git/tree/debian/patches/Use-Astropy-in-place-of-Pyfits.patch) in Debian.
