pywatchman: don't require BSERv2 for python 3 yet
pywatchman: don't require BSERv2 for python 3 yet

We can only turn this on after the server starts advertising support for
BSERv2.

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