[CI] Add Python v3.8 tests

.gitlab-ci.yml

@@ -73,6 +73,12 @@ tests-3.7:
     paths:
       - cov_html
 
+tests-3.8:
+  extends: .tests
+  image: $DOCKER_IMAGE/3.8:latest
+  script:
+    - poetry run pytest
+
 pypi_test:
   stage: publish
   when: manual