Use new python-serial API

Version 3.0 of python-serial introduced an updated API, we may as well
use it.

This change also uses context-managers for dealing with UARTs as they
are less error-prone and the code is much cleaner/shorter.

Author: sam-bristow

README.md

@@ -198,13 +198,10 @@ import serial
 
 UART.setup("UART1")
 
-ser = serial.Serial(port = "/dev/ttyO1", baudrate=9600)
-ser.close()
-ser.open()
-if ser.isOpen():
+with serial.Serial(port = "/dev/ttyO1", baudrate=9600) as ser:
     print("Serial is open!")
-    ser.write("Hello World!")
-ser.close()
+    ser.write(b"Hello World!")
+
 ```
 * Available UART names on BeagleBone
   * `UART1`: /dev/ttyO1, Rx: P9_26, Tx: P9_24

docs/UART.rst

@@ -14,13 +14,9 @@ Example::
 
     UART.setup("UART1")
 
-    ser = serial.Serial(port = "/dev/ttyO1", baudrate=9600)
-    ser.close()
-    ser.open()
-    if ser.isOpen():
-	 print "Serial is open!"
-         ser.write("Hello World!")
-    ser.close()
+    with serial.Serial(port = "/dev/ttyO1", baudrate=9600) as ser:
+        print("Serial is open!")
+        ser.write(b"Hello World!")
 
 .. module:: Adafruit_BBIO.UART