Bug: Extension schemas declaring draft-07 are validated as 2020-12

[nlebovits]

Problem

stac-validator appears to validate extension schemas as JSON Schema 2020-12 regardless of the dialect declared by the schema.

This causes valid draft-07 extension schemas to crash validation.

For example, the Portolan extension declares:

"$schema": "http://json-schema.org/draft-07/schema#"

and uses the draft-07 tuple form of items:

"items": [
  {"$ref": "#/definitions/longitude"},
  {"$ref": "#/definitions/latitude"},
  {"$ref": "#/definitions/longitude"},
  {"$ref": "#/definitions/latitude"}
]

When stac-validator reaches this definition, it crashes with:

AttributeError: 'list' object has no attribute 'get'

The relevant code appears to hardcode 2020-12:

Resource(contents=schema, specification=DRAFT202012)
validator = Draft202012Validator(...)

Under 2020-12, items expects a schema rather than the draft-07 array form, which leads referencing to call .get("$id") on the array.

Expected behavior

The validator should respect the extension schema's declared $schema dialect.

The Portolan schema validates successfully with Draft7Validator, as do all 14 reference STAC objects tested against it.

Reproduction

Schema:

https://schemas.portolan-sdi.org/portolan/v0.1.1/schema.json

Any Portolan collection that exercises the bbox definition reproduces the crash. Catalogs do not, because they do not reach that definition.

The same objects validate successfully with the draft-07 validator and with Portolan's own validator.