Replace StringIO with ByteIO

Flask on python3 won't work with StringIO as send_file only expects to be given binary file-like objects and it wouldn't know how to encode the string to bytes, according to issue #3358 in Flask repo

Signed-off-by: SolidifiedRay <[email protected]>

wizard.py

@@ -48,7 +48,7 @@
 import os
 import uuid
 import time
-import StringIO
+import io
 import tarfile
 
 from functools import wraps
@@ -1044,8 +1044,8 @@ def download_layout():
   layout_metadata = in_toto.models.metadata.Metablock(signed=layout)
 
   # Dump layout to memory file and server to user
-  layout_fp = StringIO.StringIO()
-  layout_fp.write("{}".format(layout_metadata))
+  layout_fp = io.BytesIO()
+  layout_fp.write(("{}".format(layout_metadata)).encode())
   layout_fp.seek(0)
   return send_file(layout_fp,
       mimetype="application/json", as_attachment=True,