libusb1: Use b-strings.

Removes one reason to have a version check here. This is supported since
2.6 .

Author: vpelletier

usb1/libusb1.py

@@ -142,10 +142,8 @@ def __str__(self):
 
 if sys.version_info[0] == 3:
     _string_item_to_int = lambda x: x
-    _empty_char_p = bytes()
 else:
     _string_item_to_int = ord
-    _empty_char_p = ''
 
 c_uchar = c_uint8
 c_int_p = POINTER(c_int)
@@ -737,7 +735,7 @@ class libusb_transfer(Structure):
 try:
     libusb_get_version = libusb.libusb_get_version
 except AttributeError:
-    _dummy_version = libusb_version(0, 0, 0, 0, _empty_char_p, _empty_char_p)
+    _dummy_version = libusb_version(0, 0, 0, 0, b'', b'')
     _dummy_version_p = pointer(_dummy_version)
     def libusb_get_version():
         return _dummy_version_p