py3: flush stdout
This avoids buffering due to mixed print() and sys.stdout.write() by
profiling code.
And with this, test-profile.t passes on Python 3 \o/.
Honestly, I'd be shocked if there weren't more Python 3 encoding issues lingering
in the profiling code. That code isn't super high quality and doesn't seem to have
thorough test coverage.
Differential Revision: https://phab.mercurial-scm.org/D5018
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed