ci: Bring up old trick to avoid job duplication

Author: achimnol

.travis.yml

@@ -20,17 +20,20 @@ env:
   global:
     - BACKEND_CLIENT_CLOUD_TEST=1  # use beta server for integration tests
 
+# test stage
+install:
+  - pip install -U pip setuptools
+  - pip install -U -r requirements-ci.txt
+script:
+  - python -m flake8 ai/backend tests
+  - python -m pytest --cov=ai.backend.client -v -m "not integration"
+after_success:
+  - codecov
+
+# job matrix
 jobs:
   include:
     - stage: test
-      install:
-        - pip install -U pip setuptools
-        - pip install -U -r requirements-ci.txt
-      script:
-        - python -m flake8 ai/backend tests
-        - python -m pytest --cov=ai.backend.client -v -m "not integration"
-      after_success:
-        - codecov
     - stage: deploy
       python: "3.7"
       install: skip
@@ -49,6 +52,9 @@ jobs:
     - python: "3.8"
       os: linux
   fast_finish: true
+  # exclude the duplicate default test stage
+  exclude:
+    - python: "3.6"
 
 notifications:
   webhooks: