pywatchman.pybser: add Python 3 compat imports
pywatchman.pybser: add Python 3 compat imports

Mostly this switches to using bytestrings in the right spots -- future patches
will add more compatibility with Python 3.

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