feat(ci): test on every python version

.gitlab-ci.yml

@@ -15,13 +15,16 @@ cache:
 
 test:
   stage: test
-  image: python:3.6
+  image: python:$PY_VER
   before_script:
     - pip install .[test]
   script:
     - pytest
   tags:
     - docker
+  parallel:
+    matrix:
+      - PY_VER: ["3.6", "3.7", "3.8", "3.9", "3.10"]
 
 build-linux:
   stage: build