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

tests/test-profile.t

@@ -117,11 +117,14 b' profiler extension could be loaded befor'
   $ cat > fooprof.py <<EOF
   > from __future__ import absolute_import
   > import contextlib
+  > import sys
   > @contextlib.contextmanager
   > def profile(ui, fp):
   >     print('fooprof: start profile')
+  >     sys.stdout.flush()
   >     yield
   >     print('fooprof: end profile')
+  >     sys.stdout.flush()
   > def extsetup(ui):
   >     ui.write(b'fooprof: loaded\n')
   > EOF