Python3: stbt-control-relay: Pass a unicode keyname to press()

The remote-control implementations expect to receive a native string
type: Either bytes or unicode on Python 2, or only str (unicode) on
Python 3.

This fixes `test_stbt_control_relay` on Python 3, as the
`FileControl.press()` used by the test doesn't support bytes.

stbt_control_relay.py

@@ -92,6 +92,7 @@ def main(argv):
             key = m.group("key")
             logging.debug("Received %s %s", action, key)
             try:
+                key = key.decode("utf-8")
                 if action == b"SEND_ONCE":
                     control.press(key)
                 elif action == b"SEND_START":