Ok quick update on this. That couple of hours I thought I might have didn't happen. I've taken a small peek at it now, and it is definitely not looking like a 5 minute job for me to fix which is all I have to spare right now.
The way I see it there are three options:
1. Try to make the existing NLTK Lite code Python 3 compatible - or at least the subset of it bundled with this plugin. There is only a handful of files in there, so it might not be too hard a job to do. Contributions welcome
2. Alternatively I put the whole latest NLTK library (which is huge) as part of the plugin. Not keen on doing that, am sure Kovid wouldn't want me too either.
3. Another alternative is to grab just the bits out of the latest NLTK I need. Which would require much analysis and investigation.
Then on top of that for options 2 and 3 I may also have to replace the actual analysis code, if there are compatibility issues between the old NLTK Lite and NLTK current. I found what looks to be an interesting wrapper library here for instance:
https://pypi.org/project/py-readability-metrics/
TLDR - not going to happen in the next month or so at least until my computer arrives and is operational.