python: make sure spot.automata() terminates the command

Fixes #341.

* python/spot/__init__.py (automata): Rewrite and simplify using
the subprocess context manager.
* tests/python/341.py: New file.
* tests/Makefile.am: Add it.
* NEWS: Mention the issue.

tests/python/341.py

@@ -0,0 +1,35 @@
+# -*- mode: python; coding: utf-8 -*-
+# Copyright (C) 2017 Laboratoire de Recherche et Développement de l'Epita
+# (LRDE).
+#
+# This file is part of Spot, a model checking library.
+#
+# Spot is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# Spot is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+# or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+# License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import spot
+from subprocess import _active
+
+def two_intersecting_automata():
+    """return two random automata with a non-empty intersection"""
+    g = spot.automata('randaut -A4 -Q5 -n-1 2 |')
+    for a, b in zip(g, g):
+        if a.intersects(b):
+            return a, b
+
+for i in range(5):
+    two_intersecting_automata()
+
+n = len(_active)
+print(n, "active processes")
+assert(n == 0);