use python docker for CI instead of ubuntu

.gitlab-ci.yml

@@ -1,8 +1,8 @@
 test:
-  image: ubuntu:disco
+  image: python
   script:
   - apt-get update -qy
-  - apt-get install -y python3-pip tor
+  - apt-get install -y tor
   - pip3 install --require-hashes -r requirements.txt
   - pip3 install --require-hashes -r requirements-notifications.txt
   - python3 --version