Add 'yellow' as an alias of 'amber' for TrafficLights

Fixes #345

tests/test_boards.py

@@ -551,6 +551,11 @@ def test_traffic_lights():
         assert red_pin.state
         assert not amber_pin.state
         assert not green_pin.state
+    with TrafficLights(red=red_pin, yellow=amber_pin, green=green_pin) as board:
+        board.yellow.on()
+        assert not red_pin.state
+        assert amber_pin.state
+        assert not green_pin.state
 
 def test_traffic_lights_bad_init():
     with pytest.raises(ValueError):