Python build examples contains invalid YAML

See https://computing.docs.ligo.org/gitlab-ci-templates/python/#.python:build-example, where it has the example:

tarball:
  extends:
    - .python:build
  variables:
    WHEEL: false

This gives: This GitLab CI configuration is invalid: jobs:tarball:variables config should be a hash of key value pairs.

All the booleans should be "true"/"false" strings instead in the usage and examples.