Bug#750612: gnome-music: Unneeded build-dep on python3-dev

[Scott Kitterman]

Package: gnome-music
Version: 3.12.1-2
Severity: normal

No compiled python3 extensions are part of gnome-music, so build-dep on
python3-dev is not required.  A build-dep on python3 is sufficient.

Having the build-dep be on python3-dev makes the package show up as possibly
needing a binNMU during python3 transitions (it doesn't) and makes each build
take longer since additional packages need to be installed.