Create validate_defaults function by passing in the instance_validator

The instance_validator was previously set as an attribute on the
OpenApiValidator. However with the jsonschema upgrade to 4.0.0, this
attribute is no longer maintained while descending into the spec.
Presumably because jsonschema now relies on attrs.resolve.

connexion/spec.py

@@ -6,6 +6,7 @@
 import copy
 import json
 import pathlib
+import typing as t
 from collections.abc import Mapping
 from urllib.parse import urlsplit
 
@@ -24,33 +25,37 @@
 validate_properties = Draft4Validator.VALIDATORS["properties"]
 
 
-def validate_defaults(validator, properties, instance, schema):
-    """Validate `properties` subschema.
+def create_validate_default_fn(instance_validator: Draft4Validator) -> t.Callable:
+    """Creates a validation function for property defaults. This validation function will be used
+    by a validator that validates an openapi spec against the openapi schema. The default value
+    however needs to be validated against the openapi spec itself.
+
+    :param instance_validator: A validator to validate defaults against the openapi spec itself
+                               instead of against the openapi schema.
+
+    :return: A validation function for property defaults using the passed in instance_validator
 
-    Enforcing each default value validates against the schema in which it resides.
     """
-    valid = True
-    for error in validate_properties(
-        validator, properties, instance, schema,
-    ):
-        valid = False
-        yield error
-
-    # Validate default only when the subschema has validated successfully
-    # and only when an instance validator is available.
-    if not valid or not hasattr(validator, 'instance_validator'):
-        return
-    if isinstance(instance, dict) and 'default' in instance:
-        for error in validator.instance_validator.iter_errors(
-            instance['default'],
-            instance
-        ):
+
+    def validate_defaults(validator, properties, instance, schema):
+        """Validate `properties` subschema.
+
+        Enforcing each default value validates against the schema in which it resides.
+        """
+        valid = True
+        for error in validate_properties(validator, properties, instance, schema):
+            valid = False
             yield error
 
+        # Validate default only when the subschema has validated successfully
+        if not valid:
+            return
+        if isinstance(instance, dict) and 'default' in instance:
+            for error in instance_validator.iter_errors(instance['default'], instance):
+                yield error
+
+    return validate_defaults
 
-OpenApiValidator = extend_validator(
-    Draft4Validator, {"properties": validate_defaults},
-)
 
 NO_SPEC_VERSION_ERR_MSG = """Unable to get the spec version.
 You are missing either '"swagger": "2.0"' or '"openapi": "3.0.0"'
@@ -83,8 +88,10 @@ def _validate_spec(cls, spec):
         """ validate spec against schema
         """
         try:
+            instance_validator = Draft4Validator(spec)
+            validate_defaults = create_validate_default_fn(instance_validator)
+            OpenApiValidator = extend_validator(Draft4Validator, {"properties": validate_defaults})
             validator = OpenApiValidator(cls.openapi_schema)
-            validator.instance_validator = Draft4Validator(spec)
             validator.validate(spec)
         except jsonschema.exceptions.ValidationError as e:
             raise InvalidSpecification.create_from(e)