Add `PullRequest.get_issue_timeline` method (#3259)

Currently there is already `get_issue_events` method, but
`get_issue_timeline` is missing. This means users have to use two
requests instead of one: `as_issue().get_timeline()`. Seems to be
suboptimal.

This pull request adds `PullRequest.get_issue_timeline()` in the most
straightforward way.