Allow NEOPIXEL_POWER* init outside of NeoPixel lib

Ignore exception when trying to set up `NEOPIXEL_POWER`* pin, since user may have set that up already for controlling power independently of this library.

Tested on `Adafruit CircuitPython 7.0.0-alpha.5-187-g1e53cf40a on 2021-08-11; Adafruit MagTag with ESP32S2`; code snippet:
```py
import digitalio, neopixel

# NeoPixels & light sensor
aux_power_io = digitalio.DigitalInOut(board.NEOPIXEL_POWER_INVERTED)
aux_power_io.direction = Direction.OUTPUT
aux_power_io.value = False  # active low

status_light = neopixel.NeoPixel(board.NEOPIXEL, 4, brightness=0.05, pixel_order=neopixel.GRB)
```

neopixel.py

@@ -120,8 +120,11 @@ def __init__(
             if not power:
                 power = getattr(board, "NEOPIXEL_POWER", None)
             if power:
-                self._power = digitalio.DigitalInOut(power)
-                self._power.switch_to_output(value=polarity)
+                try:
+                    self._power = digitalio.DigitalInOut(power)
+                    self._power.switch_to_output(value=polarity)
+                except ValueError:
+                    pass
 
         super().__init__(
             n, brightness=brightness, byteorder=pixel_order, auto_write=auto_write