CI: Add job to run tests on gentoo

Closes #93
9 jobs from master in 5 minutes and 40 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Linting
passed #90861
linting:bandit

00:00:17

passed #90864
linting:mypy

00:00:18

passed #90863
linting:pyflakes

00:00:16

passed #90862
linting:pylint

00:00:23

 
  Test
passed #90865
tests:archlinux

00:01:02

passed #90866
tests:debian

00:01:02

98.0%
passed #90867
tests:debian_with_bubblewrap

00:01:34

100.0%
passed #90868
tests:fedora

00:01:00

passed #90869
tests:gentoo

00:01:30