How to revert a commit of a project

## Description of the problem, including code/CLI snippet
I need to remove an specific commit in a project

## Expected Behavior
https://docs.gitlab.com/ce/api/commits.html#revert-a-commit

## Actual Behavior


## Specifications

  - python-gitlab version: 
  - API version you are using (v3/v4): v4
  - Gitlab server version (or gitlab.com): 12.4
