pywatchman: support Python 3 for tests module
pywatchman: support Python 3 for tests module

It's easiest to just commit all these changes in one go -- they include:
- adding `__future__` imports
- switching to bytestrings
- disabling some tests that don't make sense on Python 3

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