mypaint: Namespace GdkPixbuf not available

[nico202]

Issue description

mypaint does not start. Error:

Traceback (most recent call last):                                                                                                                                                               
  File "/nix/store/jljll1z7x2q0sqdkxwkx4l91xx92qppf-mypaint-1.2.1/bin/.mypaint-wrapped", line 444, in <module>
    = get_paths()
  File "/nix/store/jljll1z7x2q0sqdkxwkx4l91xx92qppf-mypaint-1.2.1/bin/.mypaint-wrapped", line 215, in get_paths
    from lib import fileutils
  File "/nix/store/jljll1z7x2q0sqdkxwkx4l91xx92qppf-mypaint-1.2.1/share/mypaint/lib/fileutils.py", line 31, in <module>
    import lib.gichecks  # this module can be imported early
  File "/nix/store/jljll1z7x2q0sqdkxwkx4l91xx92qppf-mypaint-1.2.1/share/mypaint/lib/gichecks.py", line 17, in <module>
    gi.require_version('GdkPixbuf', '2.0')
  File "/nix/store/iw3bmgis8a0wny198lz6njpj06zbx0dc-python2.7-pygobject-3.26.1/lib/python2.7/site-packages/gi/__init__.py", line 130, in require_version
    raise ValueError('Namespace %s not available' % namespace)
ValueError: Namespace GdkPixbuf not available

Steps to reproduce

nix-shell --pure -p mypaint --run mypaint

Technical details

• system: "x86_64-linux"
• host os: Linux 4.14.43, NixOS, 18.09pre140958.696c6bed4e8 (Jellyfish)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.0.2
• channels(IITaudio): "master, nixos"
• channels(gitlab-runner): "nixpkgs-18.03pre117013.aebdc892d6"
• channels(root): "nixos-18.09pre140958.696c6bed4e8"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos