Branch part of cache key

Hi, in `.circleci/config.yml`, you use `{{ .Branch }}` in addition to a checkusm of requirements.txt as part of the cache key for the python virtual environment. Could you explain the reasoning behind this? I see this is not the case for the [nodejs demo repo](https://github.com/CircleCI-Public/circleci-demo-javascript-express/blob/master/.circleci/config.yml#L14) with package.json