Run Dogtail using Python 3

Originally created by @intrigeri on #16976 (Redmine)

Currently we install python-dogtail, which uses Python 2 and makes it impossible to deal with objects containing unicode character (e.g. “Save As…” — note the triple dots) aka. https://bugs.debian.org/728658. This was originally tracked on #12185 (closed) that got repurposed in the meantime.

Debian testing (Bullseye) now has 0.9.11-2 with a python3-dogtail binary package that should be installable on Stretch and Buster, and allow us to drop some ugly hacks from our test suite.

This is part of our #15953 (closed) roadmap item.

Feature Branch: test/16004-adapt-usb-scenarios-to-usb-images+force-all-tests

Related issues

• Blocks #8788 (closed)
• Blocks #15953 (closed)
• Blocked by #12185 (closed)
• Blocked by #16004 (closed)