Fix CI

.gitlab-ci.yml

@@ -1,4 +1,4 @@
-image: python:3.6
+image: python:3.8
 
 stages:
   - test
@@ -21,6 +21,11 @@ python37:
   stage: test
   script: tox -e py37
 
+python38:
+  image: python:3.8
+  stage: test
+  script: tox -e py37
+
 linters:
   stage: test
   script: tox -e linters