text: Title: Stop using failing ujson as default serializer
  Body: ujson is no more actively maintained and specially for some critical bugs not present in json package.
  
  one example forcing us to revert ujson usage:
  https://github.com/esnme/ultrajson/issues/325 (2 days ago, mine)
  https://github.com/esnme/ultrajson/issues/301 (same but 1 year ago and no answer)
  