Move from validictory to jsonschema

[paultag]

validictory was deprecated circa 2018, James suggested https://github.com/python-jsonschema/jsonschema in its place, which I think is a good idea.

This is causing a local failure for me under Python 3.11; this is likely a good starter task for someone if it doesn't become critical and force an update by someone :)

This was discovered in #84