autotest [PY3] - fix check_control_files presubmit in py3

BUG=b:216863560
TEST=this in py3

Change-Id: If95dfd55f11f57402e7857a9c3950919eb07be4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/3429532
Reviewed-by: Curt Williams <curtwilliams@google.com>
Auto-Submit: Derek Beckett <dbeckett@chromium.org>
Reviewed-by: Jesse McGuire <jessemcguire@google.com>
Tested-by: Derek Beckett <dbeckett@chromium.org>
Commit-Queue: Derek Beckett <dbeckett@chromium.org>
diff --git a/site_utils/presubmit_hooks/check_control_files.py b/site_utils/presubmit_hooks/check_control_files.py
index 516f41c..b29e132 100755
--- a/site_utils/presubmit_hooks/check_control_files.py
+++ b/site_utils/presubmit_hooks/check_control_files.py
@@ -108,7 +108,11 @@
                     GetAutotestTestPackages(overlay))
         child = subprocess.Popen(cmd_args, stdout=subprocess.PIPE,
                                  stderr=subprocess.PIPE)
-        new_useflags = child.communicate()[0].splitlines()
+        # [bytes] ==> [str]
+        new_useflags = [
+                c.decode() if isinstance(c, bytes) else c
+                for c in child.communicate()[0].splitlines()
+        ]
         if child.returncode == 0:
             useflags = useflags.union(new_useflags)
     return useflags
@@ -288,6 +292,5 @@
             CheckRetry(ctrl_data, test_name)
             CheckDependencies(ctrl_data, test_name)
 
-
 if __name__ == '__main__':
     main()