title: How to reopen a closed Gitlab Projectissue with the python gitlab module
body: <p>Trying to automate reopening a bunch of issues on a kanban board in Gitlab does not seem to be working as expected. I can edit pretty much everything else: labels, title, description, due date and so on, but changing the state from 'closed' to 'opened' is not doing anything.
  Funny enough the save() method, takes some time to complete and is not throwing any error (but it does when I try to update an issue without having changed anything)</p>
  <pre><code>import gitlab
  
  gl = gitlab.Gitlab(URL, private_token=TOKEN, api_version = 4)
  gl.auth()
  Project = gl.projects.get(project_id, lazy=True)
  editable_issue = Project.issues.get(issue_id)
  </code></pre>
  <p>I use this to instantiate the gitlab object</p>
  <p>This works:</p>
  <pre><code>editable_issue.labels.append('some label')
  editable_issue.save()
  
  editable_issue.title = title + '\n' + 'edited.'
  editable_issue.save()
  </code></pre>
  <p>But this doesn't:</p>
  <pre><code>editable_issue.state = 'opened'
  editable_issue.save()
  </code></pre>
  <p>I have also tried changing several fields, like so:</p>
  <pre><code>editable_issue.state = 'opened'
  editable_issue.closed_at = None
  editable_issue.save()
  </code></pre>
  <p>But, albeit I receive no error, the task is not being updated.</p>
  <p>Is there something I'm overlooking ?</p>
  <p><strong>edit</strong>
  Oh yes, I forgot to mention that the documentation seems to be mentioning <code>state_event</code> as a method, while my object does not know of such a method.
  Is this method only there when a projectissue is being created by the api, perhaps ?</p>