Create the gyb output file if it doesn't exist

Python behavior changed?!

Tools/gyb.py

@@ -1254,7 +1254,7 @@ def succ(a):
         sys.stdout.write(result_text)
     else:
         os.chdir(saved_cwd)
-        with io.open(args.target, 'w', encoding='utf-8', newline='\n') as f:
+        with io.open(args.target, 'w+', encoding='utf-8', newline='\n') as f:
             f.write(result_text)