v1.2.0: Add nbestToNodeList, drop Python 3.6 and earlier

This release of fugashi adds one new feature: Tagger.nbestToNodeList returns the top N possible tokenizations of a string as node lists. Many thanks to @teowenshen for the implementation (#61).

This release also drops support for Python 3.6 and earlier versions. While the current source should still work with 3.5 and 3.6, wheels are not provided, and it is recommended you upgrade your Python version to one that has not reached end-of-life status. If you must use an older version, you can continue using v1.1.2.