Be consistent between str and decode for internal_test

We should use decode everywhere, filed b/221778256 to track this

Bug: 221778256
Change-Id: I74c1b09a71bcee652dae8316662418482c867136
diff --git a/tools/utils.py b/tools/utils.py
index 01d875c..79efd9c 100644
--- a/tools/utils.py
+++ b/tools/utils.py
@@ -365,7 +365,7 @@
   cmd = [get_gsutil(), 'cat', destination]
   PrintCmd(cmd)
   try:
-    return str(subprocess.check_output(cmd))
+    return subprocess.check_output(cmd).decode('utf-8').strip()
   except subprocess.CalledProcessError as e:
     if ignore_errors:
       return ''