Tests: USB: Fix Python 3 compatibility

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

Author: Filip Jagodzinski

TESTS/host_tests/pyusb_basic.py

@@ -1376,7 +1376,7 @@ 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

@@ -24,6 +24,7 @@
 import sys
 import serial
 import serial.tools.list_ports as stlp
+import six
 import mbed_host_tests
 
 
@@ -268,7 +269,7 @@ def change_line_coding(self):
         mbed_serial.reset_output_buffer()
         mbed_serial.dtr = True
         try:
-            payload = mbed_serial.read(LINE_CODING_STRLEN)
+            payload = six.ensure_str(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 +278,7 @@ 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 = six.ensure_str(mbed_serial.read(LINE_CODING_STRLEN))
         except serial.SerialException as exc:
             self.log('TEST ERROR: {}'.format(exc))
             self.notify_complete(False)