check_whence: python3/utf-8 support

WHENCE contains non-ASCII characters, so we should specify an encoding.
To get the encoding= arg in python2, we pull open from 'io' directly.
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Josh Boyer <jwboyer@kernel.org>

check_whence.py

 #!/usr/bin/python
 
 import os, re, sys
+from io import open
 
 def list_whence():
-    with open('WHENCE') as whence:
+    with open('WHENCE', encoding='utf-8') as whence:
         for line in whence:
             match = re.match(r'(?:File|Source):\s*"(.*)"', line)
             if match: