libusb1: Fix get_extra on python 2.7 .

Whereas single-item retrieval on a "bytes" instance yields a string and
hence requires an "ord" call, on "bytearray" instance it yields an integer
directly. This was broken by:
  commit b26f6cd
  Author: Vincent Pelletier <plr.vincent@gmail.com>
  Date:   Sun Oct 16 11:14:04 2016 +0000

      Use writeable buffer for transfers (sync & async).
along with buffer_at cast to bytearray.

Author: vpelletier

usb1/libusb1.py

@@ -140,11 +140,6 @@ def __str__(self):
         return '%s [%s]' % (libusb_error.get(self.value, 'Unknown error'),
                             self.value)
 
-if sys.version_info[0] == 3:
-    _string_item_to_int = lambda x: x
-else:
-    _string_item_to_int = ord
-
 c_uchar = c_uint8
 c_int_p = POINTER(c_int)
 
@@ -1104,7 +1099,7 @@ def get_extra(descriptor):
         extra = buffer_at(descriptor.extra, extra_length)
         append = result.append
         while extra:
-            length = _string_item_to_int(extra[0])
+            length = extra[0]
             if not 0 < length <= len(extra):
                 raise ValueError(
                     'Extra descriptor %i is incomplete/invalid' % (