Copyright for purity source package in Bionic

Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
Upstream-Name: purity
Source: ftp://metalab.unc.edu/pub/Linux/games/amusements/purity.tar.Z

Files: *
Copyright: 1997 Eric Lechner 
License: public-domain

Files: debian/*
Copyright: 1997-2001 Aaron Howell 
           2001-2002 Duncan Findlay 
           2011      Bernhard R. Link 
           2016      Giovani Augusto Ferreira 
License: public-domain

License: public-domain
 This program is public domain. Please don't charge for it.
 If you improve it/fix it/etc, please let me know, so i can
 keep my version up to date with everyone else's.