extras/servdata_consumer.py: Properly handle pre-encoded Mako output

[zmousm]

• ServdataWriter specifies output_encoding for Mako, thus render() returns PY2/PY3 str/bytes, i.e. already encoded output
• To properly handle this
  • Open output file in binary mode
  • Use sys.stdout.buffer.write() for PY3
• We can then remove the six dependency