Port tails-about to Python 3

/usr/local/bin/tails-about should be migrated to Python3/PyGI.

To test: start a Tails, select “More options”, set an admin (sudo) password, apply your changes, and then click “About Tails” in the applications menu.

Feature Branch: sycamoreone:10082-port-tails-about-to-python3

Parent Task: #5958

Original created by @alant on 10082 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information