validating nan in an enum is no longer supported

[shinnar]

As a result of the change in a9bf13c

equal (which uses ==) is now used for all enum (https://github.com/python-jsonschema/jsonschema/blob/main/jsonschema/_keywords.py#L269) validations.

As a result, if an enum has a nan in it, it will no longer validate a nan instance, since nan == nan returns False.
The previous behaviour used in, which calls the containers __contains__ method. For standard containers, that uses both is and == to check for comparison, which is why this worked before this commit.

One possible fix would be to check if one is two at the beginning of the equal method.

[Julian]

Similar to #1222 NaN doesn't exist in JSON, and therefore not in JSON Schema so it isn't really supported by this library and there's certainly no guarantees about how it behaves. That being said, I'm happy nonetheless to merge your PR, but only really because an is check might help in a tiny way for other comparison speedups. There again would be no promises about behavior of nan in particular.