Set pygi-python3 to the default branch

[xuzhen]

Since all the future development will be in that branch, it should be the default one.

[twa022]

If you do this, could you tag the commit as 0.6?
I package xfce4-dockbarx-plugin for archlinux, and it would be great to package the tagged release.
The gtk3 version is already packaged as xfce4-dockbarx-plugin-gtk3-git, but it'd be great to package this version as the release package instead of the old gtk2 version, especially if dockbarx is tagged too.