Change assert to if in get_pin()

Replace assert stmt with if stmt raising a
ValueError if pin number is out of bounds.

Author: SAK917

adafruit_mcp230xx/mcp23008.py

@@ -85,5 +85,6 @@ def get_pin(self, pin):
         """Convenience function to create an instance of the DigitalInOut class
         pointing at the specified pin of this MCP23008 device.
         """
-        assert 0 <= pin <= 7
+        if not 0 <= pin <= 7:
+            raise ValueError("Pin number must be 0-7.")
         return DigitalInOut(pin, self)