pygobject: update to 2.28.6

This fixes an issue with MyPaint: https://gna.org/bugs/?20400
Merge remote-tracking branch 'upstream/pygobject-2.28.6'

This branch updates pygobject (the diff itself is trivial), which
affects a lot of packages but due to the recent stdenv merge this seems
like a good time for this.

Signed-off-by: Shea Levy <shea@shealevy.com>