fix: Retry deleting a branch if GitHub says it's unknown

Try to fix #687 by allowing
retries on `delete_branch`. This may prevent the issue we've seen in
`create_private_to_public_pr.py` where the deletion of a newly created
branch occasionally fails with a 404.

tubular/github_api.py

@@ -664,8 +664,17 @@ def get_commits_by_branch(self, branch):
         branch = self.github_repo.get_branch(branch)
         return self.github_repo.get_commits(branch.commit.sha)
 
-    @backoff.on_exception(backoff.expo, (RateLimitExceededException, socket.timeout), max_tries=7,
-                          jitter=backoff.random_jitter, on_backoff=_backoff_logger)
+    @backoff.on_exception(
+        backoff.expo,
+        (
+            RateLimitExceededException, socket.timeout,
+            # It seems we can sometimes try to delete too
+            # quickly after having created a branch:
+            # https://github.com/openedx/tubular/issues/687
+            UnknownObjectException,
+        ),
+        max_tries=7, jitter=backoff.random_jitter, on_backoff=_backoff_logger,
+    )
     def delete_branch(self, branch_name):
         """
         Call GitHub's delete ref (branch) API