Fail to cache Python dependencies

Summary

We can't cache Python dependencies on GitLab CI even by following the procedures on the official template.

• Docs (correct): https://docs.gitlab.com/ce/ci/caching/#caching-python-dependencies
• CI Template: https://gitlab.com/gitlab-org/gitlab-ce/blob/v11.4.0-rc2/lib/gitlab/ci/templates/Python.gitlab-ci.yml

Possible fixes

Fix docs and templates for that.