CI: Add job to run tests on gentoo

Closes #93
12 jobs from master in 5 minutes and 29 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Linting
passed #90754
linting:bandit

00:00:20

passed #90757
linting:mypy

00:00:39

passed #90756
linting:pyflakes

00:00:17

passed #90755
linting:pylint

00:00:22

 
  Test
passed #90763
tests:archlinux

00:01:01

passed #90759
tests:debian

00:00:57

98.0%
passed #90764
tests:debian_with_bubblewrap

00:01:16

100.0%
passed #90765
tests:fedora

00:01:15

passed #90762
tests:gentoo

00:03:31

failed #90758
tests:archlinux

00:02:01

failed #90760
tests:debian_with_bubblewrap

00:02:03

failed #90761
tests:fedora

00:00:33