Look into showing deprecation warnings

[jspaaks]

A recent draft of JSONSchema has a key deprecated, it would be useful to annotate some keys in the schema with it, and have its corresponding comment show up when validating.

https://json-schema.org/draft/2019-09/json-schema-validation.html#rfc.section.9.3

[jspaaks]

I experimented a bit with updating the schema to 2019-09, annotating the commit key as deprecated, and then creating a CITATION.cff that uses commit. However, nothing shows up in the terminal.

Also tried with https://pypi.org/project/check-jsonschema/, that also didn't show any deprecation warnings.

Maybe there is more stuff in the Python objects, haven't looked at that yet.