py/makedefs: Use io.open with utf-8 encoding when processing source.

In case (user) source code contains utf-8 encoded data and the default
locale is not utf-8.

See #4592.

py/makeqstrdefs.py

@@ -9,6 +9,7 @@ from __future__ import print_function
 
 import re
 import sys
+import io
 import os
 
 # Blacklist of qstrings that are specially handled in further
@@ -108,7 +109,7 @@ if __name__ == "__main__":
         pass
 
     if args.command == "split":
-        with open(args.input_filename) as infile:
+        with io.open(args.input_filename, encoding='utf-8') as infile:
             process_file(infile)
 
     if args.command == "cat":