Decode stderr after executing a replay

The touch tests were showing stderr as a binary string when run in
Python 3,

BUG=none
TEST=run an erroring touch test using Python 3 and check the stderr

Change-Id: I9f547766dffa99e7be4c2a5ed6b6442524aeb482
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/mttools/+/2166455
Tested-by: Harry Cutts <hcutts@chromium.org>
Auto-Submit: Harry Cutts <hcutts@chromium.org>
Reviewed-by: Sean O'Brien <seobrien@chromium.org>
Commit-Queue: Harry Cutts <hcutts@chromium.org>
diff --git a/mtreplay/mtreplay.py b/mtreplay/mtreplay.py
index 98cbe19..ef2b1c2 100755
--- a/mtreplay/mtreplay.py
+++ b/mtreplay/mtreplay.py
@@ -321,6 +321,6 @@
       print('Gestures Log: ')
       print(self.gestures_log)
       print('Standard error:')
-      print(process.stderr.read())
+      print(process.stderr.read().decode(errors='replace'))
       raise Exception('Error {} from process: {}'
               .format(process.returncode, ' '.join(parameters)))