avoid python 2.7+ set literals
avoid python 2.7+ set literals

Summary:
We still need to run with python 2.6
Closes https://github.com/facebook/watchman/pull/581

Differential Revision: D7190828

Pulled By: wez

fbshipit-source-id: fc037ae7d4ea8b50c6524f15ecc17014d57f0297

wez/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,wez/watchman,wez/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,wez/watchman,wez/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,wez/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,wez/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,wez/watchman,wez/watchman,facebook/watchman