Fix dex-segment utils script for running on python3

Change-Id: Iaaa59c4b8f44522e84aca8fc057428d17b46cbd5
diff --git a/tools/utils.py b/tools/utils.py
index a55ea58..dcfa4f8 100644
--- a/tools/utils.py
+++ b/tools/utils.py
@@ -455,7 +455,7 @@
   cmd = [jdk.GetJavaExecutable(), '-jar', R8_JAR, 'dexsegments']
   cmd.extend(dex_files)
   PrintCmd(cmd)
-  output = subprocess.check_output(cmd)
+  output = subprocess.check_output(cmd).decode('utf-8')
 
   matches = DEX_SEGMENTS_RESULT_PATTERN.findall(output)