Upstream links changed

Hi, I am the author of pngtools. After prompting from Arch Linux, I've recovered the source code for pngtools from a backup and placed it on github at https://github.com/mikalstill/pngtools.

I'd be happy enough to accept patches and bug reports there, and will do my best to service them -- noting that this code is nearly 20 years old.

I'm not sure what the correct Ubuntu process is for such things, so I am just filing this bug. As best as I can tell the upstream package is unmaintained in Debian.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: pngtools 0.4-1.3
ProcVersionSignature: Ubuntu 5.4.0-58.64-generic 5.4.73
Uname: Linux 5.4.0-58-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.14
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Sun Dec 27 14:27:54 2020
InstallationDate: Installed on 2020-08-29 (119 days ago)
InstallationMedia: Ubuntu 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
SourcePackage: pngtools
UpgradeStatus: No upgrade log present (probably fresh install)