Fix #3266 and CVE-39705, by using the RestrictedUnpickler class from nltk.app.wordnet_app, to prevent from loading pickled classes or functions.

For example:

from nltk.data import load
from nltk.corpus import treebank

pos = load("taggers/maxent_treebank_pos_tagger/PY3/english.pickle")
print(pos.tag(treebank.sents()[0]))

Without this PR, the pickle loads and works:

[('Pierre', 'NNP'), ('Vinken', 'NNP'), (',', ','), ('61', 'CD'), ('years', 'NNS'), ('old', 'JJ'), (',', ','), ('will', 'MD'), ('join', 'VB'), ('the', 'DT'), ('board', 'NN'), ('as', 'IN'), ('a', 'DT'), ('nonexecutive', 'JJ'), ('director', 'NN'), ('Nov.', 'NNP'), ('29', 'CD'), ('.', '.')]

With this PR, loading classes or functions raise an error:

_pickle.UnpicklingError: global 'nltk.tag.sequential.ClassifierBasedPOSTagger' is forbidden