subprocess: do not keep reference longer than necessary

gevent/subprocess.py

@@ -154,7 +154,7 @@ def check_output(*popenargs, **kwargs):
     if 'stdout' in kwargs:
         raise ValueError('stdout argument not allowed, it will be overridden.')
     process = Popen(stdout=PIPE, *popenargs, **kwargs)
-    output, unused_err = process.communicate()
+    output = process.communicate()[0]
     retcode = process.poll()
     if retcode:
         cmd = kwargs.get("args")