Version 3.5
* add support for Python 3.8
* drop support for Python 2
* create NLTK's own Tokenizer class distinct from the Treebank reference tokeniser
* update Vader sentiment analyser
* fix JSON serialization of some PoS taggers
* minor improvements in grammar.CFG, Vader, pl196x corpus reader, StringTokenizer
* change implementation <= and >= for FreqDist so they are partial orders
* make FreqDist iterable
* correctly handle Penn Treebank trees with a unlabeled branching top node.
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