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

Also fix up places in which we should be using bytestrings.

Preparing for porting to Python 3.

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