Some tests calling spot.automaton('non-existing|') where failing either
with a "process returned 127", or, under heavier load, with "failed to
read from...".  The latter occur if we poll() the exit status before the
children has had the tame to finish.

* wrap/python/spot.py: Make sure we wait for the child process if we
reach EOF, so that we can report the error status.
* wrap/python/tests/automata-io.ipynb, wrap/python/tests/piperead.ipynb:
Update.