Update gitlab-markup to 1.7.0 and document Python3 requirements

What does this MR do?

• Updates gitlab-markup to 1.7.0 that requires python3 to be available.
• Declares python dependencies in Pipfile
• Document how to use pyenv and pipenv to handle python and pip dependencies for development purpose.

EE Backport : https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10390

After merging this one, please merge omnibus-gitlab!3115 (merged)

What are the relevant issue numbers?

Closes #59062 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created