[CI] Change only/except to rules

- Will be removed from GitLab v13.0
- https://docs.gitlab.com/ee/ci/yaml/#rules
6 jobs for silkaj-0.8-dev_snapshot in 2 minutes and 53 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Checks
passed #40024
build

00:00:32

passed #40025
format

00:00:28

 
  Tests
passed #40026
poetry-56
tests-3.5

00:02:15

passed #40027
poetry-56
tests-3.6

00:02:20

passed #40028
poetry-78
tests-3.7-coverage

00:02:15

passed #40029
poetry-78
tests-3.8

00:02:06