[ci] #401: Add isort job

isort installed on 3.8 image

.gitlab-ci.yml

@@ -37,6 +37,13 @@ format:
   script:
     - black --check silkaj tests
 
+isort:
+  extends:
+    - .code_changes
+  stage: checks
+  script:
+    - isort --check silkaj tests
+
 .tests:
   extends: .changes
   stage: tests