After reading the source code:

def delete_namespaced_stateful_set(self, name, namespace, **kwargs):
    """
    delete a StatefulSet
    This method makes a synchronous HTTP request by default. To make an
    asynchronous HTTP request, please pass async_req=True
    >>> thread = api.delete_namespaced_stateful_set(name, namespace, async_req=True)
    >>> result = thread.get()
    :param async_req bool
    :param str name: name of the StatefulSet (required)
    :param str namespace: object name and auth scope, such as for teams and projects (required)
    :param str pretty: If 'true', then the output is pretty printed.
    :param V1DeleteOptions body:
    :param str dry_run: When present, indicates that modifications should not be persisted. An invalid or unrecognized dryRun directive will result in an error response and no further processing of the request. Valid values are: - All: all dry run stages will be processed
    :param int grace_period_seconds: The duration in seconds before the object should be deleted. Value must be non-negative integer. The value zero indicates delete immediately. If this value is nil, the default grace period for the specified type will be used. Defaults to a per object value if not specified. zero means delete immediately.
    :param bool orphan_dependents: Deprecated: please use the PropagationPolicy, this field will be deprecated in 1.7. Should the dependent objects be orphaned. If true/false, the \"orphan\" finalizer will be added to/removed from the object's finalizers list. Either this field or PropagationPolicy may be set, but not both.
    :param str propagation_policy: Whether and how garbage collection will be performed. Either this field or OrphanDependents may be set, but not both. The default policy is decided by the existing finalizer set in the metadata.finalizers and the resource-specific default policy. Acceptable values are: 'Orphan' - orphan the dependents; 'Background' - allow the garbage collector to delete the dependents in the background; 'Foreground' - a cascading policy that deletes all dependents in the foreground.
    :return: V1Status
             If the method is called asynchronously,
             returns the request thread.
    """

I believe the correct way to set the extra params (**kwargs) is using the key=value format, also V1DeleteOptions is not a valid parameter so instead of passing delete_options as an argument you should pass each option of V1DeleteOptions, such as propagation_policy="Foreground" and grace_period_seconds=5:

api_response = k8s_api.delete_namespaced_stateful_set(statefulset_name,namespace,propagation_policy="Foreground", grace_period_seconds=5)

There is a way, however, to pass an entire V1DeleteOptions object. You can first convert it to a dict and then unpack it:

delete_options = client.V1DeleteOptions(propagation_policy="Foreground", grace_period_seconds=5)
api_response = k8s_api.delete_namespaced_stateful_set(statefulset_name,namespace,**delete_options.do_dict())