change parser

[aoifecahill]

python-zpar is no longer being actively maintained so we should try and find an alternative dependency, e.g. https://demo.allennlp.org/constituency-parsing

Or if constituencies are not actually needed, the spacy dep parser or the stanford stanza dep parser might be good options.

[desilinguist]

Note that until we do this, the discourse parser will pretty much be Linux only since python-zpar no longer compiles on a Mac.