Tests: USB: Fix Python 3 compatibility

Use integer division explicitly in basic test.
Convert bytes to str for Python 3 in serial test.

TESTS/host_tests/pyusb_basic.py

@@ -1369,7 +1369,7 @@ def ep_test_abort(dev, log, verbose=False):
             payload_size = (NUM_PACKETS_UNTIL_ABORT + NUM_PACKETS_AFTER_ABORT) * ep_out.wMaxPacketSize
             num_bytes_written = 0
             while num_bytes_written < payload_size:
-                payload_out = array.array('B', (num_bytes_written/ep_out.wMaxPacketSize
+                payload_out = array.array('B', (num_bytes_written//ep_out.wMaxPacketSize
                                                 for _ in range(ep_out.wMaxPacketSize)))
                 try:
                     num_bytes_written += ep_out.write(payload_out)

TESTS/host_tests/usb_device_serial.py

@@ -64,6 +64,16 @@ def usb_serial_name(serial_number):
     return None
 
 
+if sys.version_info < (3,):
+    def bytes2str(source):
+        """Return a string decoded from source."""
+        return source
+else:
+    def bytes2str(source):
+        """Return a string decoded from source."""
+        return source.decode()
+
+
 class RetryError(Exception):
     """Exception raised by retry_fun_call()."""
 
@@ -268,7 +278,7 @@ def change_line_coding(self):
         mbed_serial.reset_output_buffer()
         mbed_serial.dtr = True
         try:
-            payload = mbed_serial.read(LINE_CODING_STRLEN)
+            payload = bytes2str(mbed_serial.read(LINE_CODING_STRLEN))
             while len(payload) == LINE_CODING_STRLEN:
                 baud, bits, parity, stop = (int(i) for i in payload.split(','))
                 new_line_coding = {
@@ -277,7 +287,7 @@ def change_line_coding(self):
                     'parity': self._PARITIES[parity],
                     'stopbits': self._STOPBITS[stop]}
                 mbed_serial.apply_settings(new_line_coding)
-                payload = mbed_serial.read(LINE_CODING_STRLEN)
+                payload = bytes2str(mbed_serial.read(LINE_CODING_STRLEN))
         except serial.SerialException as exc:
             self.log('TEST ERROR: {}'.format(exc))
             self.notify_complete(False)