Define xrange for use under Python 3

Define xrange() for Python 3. I is only used on for loops so this is safe.

cpplint/cpplint.py

@@ -52,6 +52,11 @@ import string
 import sys
 import unicodedata
 
+try:
+  xrange          # Python 2
+except NameError:
+  xrange = range  # Python 3
+
 
 _USAGE = """
 Syntax: cpplint.py [--verbose=#] [--output=vs7] [--filter=-x,+y,...]

cpplint/cpplint_unittest.py

@@ -43,6 +43,11 @@ import unittest
 
 import cpplint
 
+try:
+  xrange          # Python 2
+except NameError:
+  xrange = range  # Python 3
+
 
 # This class works as an error collector and replaces cpplint.Error
 # function for the unit tests.  We also verify each category we see