Copyright for python-merge3 source package in Mantic

Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
Upstream-Name: merge3
Upstream-Contact: Breezy-Developers 
Source: https://github.com/breezy-team/merge3

Files: *
Copyright:
  2005-2010 Canonical Ltd
License: GPL-2+

Files: debian/*
Copyright: 2020 Jelmer Vernooij 
License: GPL-2+

License: GPL-2+
 On Debian systems the full text of the GNU General Public License version 2
 can be found in the `/usr/share/common-licenses/GPL-2' file.