title: Return only issues using PyGithub
body: <p>I'm trying to get the number of issues in a repository, but the code below is returning issues and pull requests.  How do I just get issues?  I think I'm missing something simple here.  I read the api documentation and it states GitHub treats all pull requests as issues.</p>
  
  <pre><code>repo = g.get_repo("someRepo")
  label = repo.get_label('someLabel')
  myIssues = repo.get_issues(state='open',labels=[label])
  count = 0
  for issue in myIssues:
      count = count + 1
  print(count)
  </code></pre>