PopenProcess: use call_soon for _async_waipid in _start

Use call_soon to delay the add_child_handler call that
_async_waipid will trigger, so that it will occur after
the event loop is running.

Bug: https://bugs.gentoo.org/770712
Signed-off-by: Zac Medico <zmedico@gentoo.org>
1 file changed