Don't use outdated Docker image

.gitlab-ci.yml

 upload_to_pypi:
-  image: pypi/twine:latest
+  image: python:3-alpine
   stage: deploy
   only:
     - tags
   tags:
     - dbnomics-python-client
   before_script:
+    - pip install twine
     - python setup.py sdist bdist_wheel
   variables:
     TWINE_USERNAME: cbenz