Unable to render examples with only readonly properties

[sevdog]

When using the old renderer (since I cannot use the new one due to #124 and I want to keep API docs splitted based on path) with OpenAPI 3.1 and examples enabled there is a problem due to how examples are rendered and guessed:

openapi/sphinxcontrib/openapi/openapi31.py

Lines 214 to 236 in e62e298

	if examples is None:
	examples = {}
	if not example:
	if re.match(r"application/[a-zA-Z\+]*json", content_type) is None:
	LOG.info("skipping non-JSON example generation.")
	continue
	example = _parse_schema(content["schema"], method=method)
	if method is None:
	examples["Example response"] = {
	"value": example,
	}
	else:
	examples["Example request"] = {
	"value": example,
	}
	for example in examples.values():
	# According to OpenAPI v3 specs, string examples should be left unchanged
	if not isinstance(example["value"], str):
	example["value"] = json.dumps(
	example["value"], indent=4, separators=(",", ": ")
	)

If no example is provided in the swagger file one is guessed from _parse_schema, but if there are only readonly properties this method returns an object (_READONLY_PROPERTY) which is not serializable. This results in an unhandled exception (TypeError: Object of type object is not JSON serializable) which is not what is intended.