decorator @py3_data for use with the file pointers in data.py

* for use on the initializers, e.g. BufferedGzipFile
* adds PY3/ to the data path for the following datasets:

  chunkers/maxent_ne_chunker
  help/tagsets
  taggers/maxent_treebank_pos_tagger
  tokenizers/punkt

cf https://github.com/nltk/nltk/pull/359