[Request] Doctest highlighting for Python

**Is your request related to a specific problem you're having?**

I'm frustrated when syntax highlighers ignore doctests in Python docstrings. When working in gitlab its hard for people to review doctests I write because the syntax isn't highlighted. I understand that this library is what gitlab uses for highlighting, so implementing support here should solve that issue.

**The solution you'd prefer / feature you'd like to see added...**

I would like to request support for highlighting doctest syntax in docstrings.

This feature is being added to rouge-ruby: https://github.com/rouge-ruby/rouge/pull/1932 and has been supported by vim for years: