Run tests in multiple Python versions

.gitlab-ci.yml

 # Official language image. Look for the different tagged releases at:
 # https://hub.docker.com/r/library/python/tags/
-image: ${CI_DEPENDENCY_PROXY_DIRECT_GROUP_IMAGE_PREFIX}/python:3.6
+image: ${CI_DEPENDENCY_PROXY_DIRECT_GROUP_IMAGE_PREFIX}/python:latest
 
 # Change pip's cache directory to be inside the project directory since we can
 # only cache local items.
@@ -15,7 +15,6 @@ variables:
 cache:
   paths:
     - .cache/pip
-    - venv/
 
 before_script:
   - pip install virtualenv
@@ -41,6 +40,63 @@ unit-test-job:
     reports:
       junit: nose2-junit.xml
 
+unit-test-2.7-job:
+  image: ${CI_DEPENDENCY_PROXY_DIRECT_GROUP_IMAGE_PREFIX}/python:2.7
+  stage: test
+  script:
+    - make test
+
+unit-test-3.7-job:
+  image: ${CI_DEPENDENCY_PROXY_DIRECT_GROUP_IMAGE_PREFIX}/python:3.7
+  stage: test
+  script:
+    - make test 2>&1 | tee errors-3.7.log
+  artifacts:
+    when: always
+    paths:
+      - errors-3.7.log
+
+unit-test-3.8-job:
+  image: ${CI_DEPENDENCY_PROXY_DIRECT_GROUP_IMAGE_PREFIX}/python:3.8
+  stage: test
+  script:
+    - make test 2>&1 | tee errors-3.8.log
+  artifacts:
+    when: always
+    paths:
+      - errors-3.8.log
+
+unit-test-3.9-job:
+  image: ${CI_DEPENDENCY_PROXY_DIRECT_GROUP_IMAGE_PREFIX}/python:3.9
+  stage: test
+  script:
+    - make test 2>&1 | tee errors-3.9.log
+  artifacts:
+    when: always
+    paths:
+      - errors-3.9.log
+
+check-deprecation-warnings-3.7:
+  before_script: []
+  stage: check-warnings
+  script:
+    - "if [[ $(grep DeprecationWarning errors-3.7.log) ]]; then cat errors-3.7.log; exit 1; fi"
+  allow_failure: true
+
+check-deprecation-warnings-3.8:
+  before_script: []
+  stage: check-warnings
+  script:
+    - "if [[ $(grep DeprecationWarning errors-3.8.log) ]]; then cat errors-3.8.log; exit 1; fi"
+  allow_failure: true
+
+check-deprecation-warnings-3.9:
+  before_script: []
+  stage: check-warnings
+  script:
+    - "if [[ $(grep DeprecationWarning errors-3.9.log) ]]; then cat errors-3.9.log; exit 1; fi"
+  allow_failure: true
+
 pylint-test-job:
   stage: test
   script: