How to find all Pleiades URIs that correspond to a Trismegistos place URI

How to find all Pleiades URIs that correspond to a Trismegistos place URI

Python 3.6. This code assumes that you have already downloaded the Pleiades comprehensive nightly JSON export from http://atlantides.org/downloads/pleiades/json/pleiades-places-latest.json.gz and un-gzipped it in your working directory.