docs: Write files in binary mode

For python3, the mode for writing the file is required.

Change-Id: Idc17e36bb88c786027c0d6476b54844ae5d7f81e

doc/source/_exts/metadata-docs-rhel7.py

@@ -201,7 +201,7 @@ def write_file(filename, content):
     if not os.path.isdir(os.path.dirname(file_path)):
         os.makedirs(os.path.dirname(file_path))
 
-    with open(file_path, 'w') as f:
+    with open(file_path, 'wb') as f:
         f.write(content.encode('utf-8'))
 
     return True