Add DELETE to allowed methods to enable management of WebHooks via the Subscriptions API

[ghost]

    elif method == 'DELETE':
        request_params = {k:v for k,v in {
            "params": params,
            "headers": headers,
            "json": json,
            "data": body if json is None else None, }.items() if v != None}
        response = self.session.delete(url, **request_params)

this small patch to walmart.py in send_request can allow additional functionality

using it like this
r = w.send_request( 'DELETE',
'https://marketplace.walmartapis.com/v3/webhooks/subscriptions/{}'.format(subscription['subscriptionId']),
request_headers={"Content-type":"application/json","Accept":"application/json"})