Use issues endpoints for pull requests

Setting labels and assignee on Pull requests failed silently, because the API endpoint being hit contained '/pulls/' rather than '/issues/'.


> "Every pull request is an issue, but not every issue is a pull request. For this reason, “shared” actions for both features, like manipulating assignees, labels and milestones, are provided within the Issues API."

https://developer.github.com/v3/pulls/#labels-assignees-and-milestones