NB: @stevenbird, @alvations, @purificant, this PR is critical because it is not possible to test the new packages without an updated package index.
I used nltk v. 3.8.1 to make pkg_index, and it complains that it leaves 3 packages out of the index because they lack an '.xml' file: listing.csv, ptb3 and tagsets_json.
The first two seem ok to leave out, but I'm not sure about tagsets_json. The corresponding pickles in help/tagsets.zip contain only simple types, and load ok with the RestrictedUnpickler.
Anyway, adding the missing tagsets_json.xml file can only help.