Don't blow up if an automatically retrieved resource is missing $schema.

This should possibly default to the specification that the child resource
is contained within, but that's actually nontrivial to implement...

CHANGELOG.rst

@@ -3,6 +3,9 @@ v4.19.0
 
 * Importing the ``Validator`` protocol directly from the package root is deprecated.
   Import it from ``jsonschema.protocols.Validator`` instead.
+* Automatic retrieval of remote references (which is still deprecated) now properly succeeds even if the retrieved resource does not declare which version of JSON Schema it uses.
+  Such resources are assumed to be 2020-12 schemas.
+  This more closely matches the pre-referencing library behavior.
 
 v4.18.6
 =======

jsonschema/tests/test_deprecations.py

@@ -381,10 +381,7 @@ def test_automatic_remote_retrieval(self):
         Automatic retrieval of remote references is deprecated as of v4.18.0.
         """
         ref = "http://bar#/$defs/baz"
-        schema = {
-            "$schema": "https://json-schema.org/draft/2020-12/schema",
-            "$defs": {"baz": {"type": "integer"}},
-        }
+        schema = {"$defs": {"baz": {"type": "integer"}}}
 
         if "requests" in sys.modules:  # pragma: no cover
             self.addCleanup(

jsonschema/validators.py

@@ -119,7 +119,10 @@ def _warn_for_remote_retrieve(uri: str):
             DeprecationWarning,
             stacklevel=9,  # Ha ha ha ha magic numbers :/
         )
-        return referencing.Resource.from_contents(json.load(response))
+        return referencing.Resource.from_contents(
+            json.load(response),
+            default_specification=referencing.jsonschema.DRAFT202012,
+        )
 
 
 _REMOTE_WARNING_REGISTRY = SPECIFICATIONS.combine(