media-sound/gnome-music: fix python and introspection deps handling
Convert from python-r1 to python-single-r1 while we don't have any gnome-music module
consumers than gnome-music itself, to not have useless site-packages of these.
Upstream has a hardcoded to python3 shebang, so force it to what python-single-r1
ended up choosing.
Remove ugly upstream 3.4 site-packages path inserts that would potentially defeat our
whole purpose of choosing 3.5 or 3.6 over 3.4 as python provider.
Gentoo-bug: 612380
Thanks-to: Mike Gilbert <floppym@gentoo.org>