Incorrect SchemaError for enum schema with unique elements

# Draft7 for convenience; the same applies to 3, 4, and 6.
# We can have a unique list of things that compare equal, e.g. this works:
jsonschema.Draft7Validator.check_schema({'enum': [False, 0.0]})

# But wrap them in a one-element list, and we get a `SchemaError` instead!
jsonschema.Draft7Validator.check_schema({'enum': [[False], [0.0]]})

There are IMO two bugs here:

1. JSON values are being tested for equality using Python object semantics, rather than comparing them as JSON (e.g. via a canonical encoding).

2. The json-schema specification says

    The value of this keyword MUST be an array.
    This array SHOULD have at least one element.
    Elements in the array SHOULD be unique.
   

   Validation therefore ought to allow schema authors to use empty or non-unique arrays, because the RFC 2119 definition of SHOULD explicitly permits ignoring such clauses after considering the circumstances.

[erohmensing]

@Julian thanks for the “for archeological purposes”, rest assured it’s worth adding as people still run into this strange mismatch years later 😄

[Julian]

I mentioned this briefly on https://github.com/json-schema-org/json-schema-spec/issues/717#issuecomment-1299112660 but just for archaeological purposes, this change was just partially “backed out” for drafts 3 and 4 where indeed the original behavior (MUST) was correct, those drafts contain stronger language. It’s definitely correct on newer ones though (so thanks again for filing it!)