scripts/avocado: give bytes to os.write() instead of string

Python 3 is more strict about I/O to files not openend in text
mode. Let's encode the message in utf-8 and have bytes written
to the traceback log file.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

scripts/avocado

@@ -45,7 +45,7 @@ def handle_exception(*exc_info):
     except:
         pass
     tmp, name = tempfile.mkstemp(".log", prefix, crash_dir)
-    os.write(tmp, msg)
+    os.write(tmp, msg.encode('utf-8'))
     os.close(tmp)
     # Print friendly message in console-like output
     msg = ("Avocado crashed unexpectedly: %s\nYou can find details in %s"