Refactor CI configuration

.gitlab-ci.yml

-before_script:
-  - python --version
-  - pip install --upgrade --upgrade-strategy=eager pip setuptools wheel
-  - pip install -e .[test]
+default:
+  image: python:${PYTHON_VERSION}-slim-${DEBIAN_VERSION}
+  before_script:
+    - python --version
+    - pip install --upgrade --upgrade-strategy=eager pip setuptools wheel
+    - pip install -e .[test]
 
-.pytest:
+pytest:
   stage: test
   script:
     - pytest --cov-report=term-missing --junitxml=pytest.xml
@@ -11,23 +13,11 @@ before_script:
   artifacts:
     reports:
       junit: pytest.xml
-
-pytest:3.9:
-  extends: .pytest
-  image: python:3.9-slim-buster
-
-pytest:3.8:
-  extends: .pytest
-  image: python:3.8-slim-buster
-
-pytest:3.7:
-  extends: .pytest
-  image: python:3.7-slim-stretch
-
-pytest:3.6:
-  extends: .pytest
-  image: python:3.6-slim-stretch
-
-pytest:3.5:
-  extends: .pytest
-  image: python:3.5-slim-stretch
+  parallel:
+    matrix:
+      - PYTHON_VERSION: ['3.5']
+        DEBIAN_VERSION: ['jessie', 'stretch', 'buster']
+      - PYTHON_VERSION: ['3.6', '3.7']
+        DEBIAN_VERSION: ['stretch', 'buster', 'bullseye']
+      - PYTHON_VERSION: ['3.8', '3.9', '3.10']
+        DEBIAN_VERSION: ['buster', 'bullseye']