Check for the "deprecated" keyword

[edgarrmondragon]

Closes #1170

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📚 Documentation preview 📚: https://python-jsonschema--1171.org.readthedocs.build/en/1171/

[Julian]

Let's discuss this on #1170, which I may not have time to write a longer response on now but will try to tomorrow. Broadly though, I wouldn't merge this PR for a few reasons (likely additional to what I'll write there)

• the Validator protocol is for general non-dialect-specific behavior. Meaning things that go there should apply to any conceivable JSON Schema dialect -- including ones that have no deprecated keyword whatsoever.
• I won't add boolean flags to any API
• There's an existing extend API for adding behavior to a dialect (which could be used to control whether any behavior is assigned to the keyword or not) without needing to add something specifically new

I'll put some additional thoughts on the issue.

And regardless, thanks for the suggestion!