Update tests for FreeBSD and make these more visible by adding a badge (#841)

* Update tests for FreeBSD

* Use more recent images (12.2- and 13.0-RELEASE because 12.1-RELEASE is End-of-Life since 31st January 2021)
* Test against Python 3.8 which is the default in the FreeBSD ports tree at the moment

* Add a badge for FreeBSD tests

* Make the FreeBSD tests visible (where Cirrus CI still needs to be set up for the repository)
2 files changed