Replace subprocess.run with Python 2 equivalent

mailman_api/mailman.py

@@ -34,13 +34,11 @@ mm_env['LC_ALL'] = 'C'
 def _run(cmd):
     app.logger.info('running %s', cmd)
     try:
-        subprocess.run(
-            cmd, check=True, env=mm_env,
-            stdout=subprocess.PIPE,
-            stderr=subprocess.STDOUT)
+        stderr = subprocess.check_output(
+            cmd, env=mm_env, stderr=subprocess.STDOUT)
     except subprocess.CalledProcessError as e:
         app.logger.error('command FAILED (status=%d), output:\n%s\n',
-                         e.returncode, e.stdout.decode('utf-8'))
+                         e.returncode, stderr.decode('utf-8'))
         raise