unix: fix exception catch on 3.x.

mitogen/unix.py

@@ -52,7 +52,7 @@ def is_path_dead(path):
         s.connect(path)
     except socket.error:
         e = sys.exc_info()[1]
-        return e[0] in (errno.ECONNREFUSED, errno.ENOENT)
+        return e.args[0] in (errno.ECONNREFUSED, errno.ENOENT)
     return False