Relates jvoisin/mat2#130

It seems, despite the name, both packages depend on Python 3. However,
pylint3 seems deprecated, and upstream recommends to install pylint:
https://www.pylint.org/#install

The current versions of both packages in Debian unstable are:
pylint  2.4.4-1
pylint3 2.2.2-1

This commit fixes failing CI jobs due to the use of pylint3 2.2.2-1,
which seems broken.

gentoo docker image is fixed upstream
see https://github.com/gentoo/gentoo-docker-images/issues/69

this might break again if the upstream gentoo/portage image gets fixed

This is required to add svg support to mat2

Thanks, @poncho!

Closes #1

Due to a regression, gdk-pixbuff is currently broken in testing.
Accordingly, use stable. At the same time, bubblewrap is only available
in testing, accordingly, pull it from there.

Thanks to Poncho for debugging!

Signed-off-by: georg <georg@riseup.net>

Relates #1