tests: check generated PDF using bytes (#36515)

tests/test_workflows.py

@@ -2898,7 +2898,7 @@ def test_transform_to_pdf():
     instream = open(os.path.join(os.path.dirname(__file__), 'template.odt'), 'rb')
     outstream = transform_to_pdf(instream)
     assert outstream is not False
-    assert outstream.read(10).startswith('%PDF-')
+    assert outstream.read(10).startswith(b'%PDF-')
 
 def test_export_to_model_image(pub):
     formdef = FormDef()