pywatchman: add Python 3 compatibility imports to capabilities.py
pywatchman: add Python 3 compatibility imports to capabilities.py

We'll still need to handle bytestrings appropriately, but we can defer that to
later patches.

wez/watchman,wez/watchman,kwlzn/watchman,kwlzn/watchman,wez/watchman,dhruvsinghal/watchman,wez/watchman,dhruvsinghal/watchman,wez/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,dhruvsinghal/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,wez/watchman,dhruvsinghal/watchman,facebook/watchman,dhruvsinghal/watchman,dhruvsinghal/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,wez/watchman,nodakai/watchman,kwlzn/watchman,kwlzn/watchman,wez/watchman,dhruvsinghal/watchman,kwlzn/watchman,facebook/watchman,nodakai/watchman,kwlzn/watchman,facebook/watchman,dhruvsinghal/watchman,facebook/watchman,kwlzn/watchman,wez/watchman,facebook/watchman