issue #426: fix 2->3 issue in plain_old_module.

tests/data/plain_old_module.py

@@ -12,7 +12,7 @@ class MyError(Exception):
 
 def get_sentinel_value():
     # Some proof we're even talking to the mitogen-test Docker image
-    return open('/etc/sentinel').read().decode()
+    return open('/etc/sentinel', 'rb').read().decode()
 
 
 def add(x, y):