Only backport if the newly added label is the 'needs backport' label. (…

…GH-36)

Fixes python/miss-islington#35

backport/backport_pr.py

@@ -23,8 +23,11 @@ async def backport_pr(event, gh, *args, **kwargs):
 
         gh_issue = await gh.getitem(event.data['repository']['issues_url'],
                                              {'number': f"{event.data['pull_request']['number']}"})
-
-        pr_labels = await gh.getitem(gh_issue['labels_url'])
+        pr_labels = []
+        if event.data['action'] == 'labeled':
+            pr_labels = [event.data["label"]]
+        else:
+            pr_labels = await gh.getitem(gh_issue['labels_url'])
         branches = [label['name'].split()[-1]
                     for label in pr_labels
                         if label['name'].startswith("needs backport to")]