The new version requires `python3` to be available. In omnibus
installation this is already a reality as we are currently pathing
previous `gitlab-markup` version to use `python3` instead of `python2`.
We are now requiring `python3` with the gem without having to patch it.
As a consequence to also make it easy to use it in development, we've
introduced a `Pipfile` and `Pipfile.lock`, working similarly to
`Gemfile` and `Gemfile.lock`, and added documentation on how to use
them.
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