tests: use bytes for attachments (#36515)

tests/test_formdef.py

@@ -9,7 +9,7 @@ import time
 
 import pytest
 
-from django.utils.six import StringIO
+from django.utils.six import BytesIO
 from quixote import cleanup
 from wcs import fields
 from wcs.formdef import FormDef, get_formdefs_of_all_kinds
@@ -353,7 +353,7 @@ def test_unused_file_removal_job(pub):
         formdata.store()
 
         formdata.evolution[-1].parts = [AttachmentEvolutionPart('hello.txt',
-            fp=StringIO('hello world'), varname='testfile')]
+            fp=BytesIO(b'hello world'), varname='testfile')]
         formdata.store()
         assert len(glob.glob(os.path.join(pub.app_dir, 'attachments', '*/*'))) == 1
         clean_unused_files(pub)