remote-helpers: use python directly

Remote helpers use 'env python', not PYTHON_PATH, so that's where we
should check the extensions.

Suggested by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
remote-hg: more info

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>