Get the last modification date of the ontology.

def _get_last_modification_date(url):
    """Get the last modification date of the ontology."""
    request = requests.head(url)
    date_string = request.headers["last-modified"]
    parsed = time.strptime(date_string, "%a, %d %b %Y %H:%M:%S %Z")
    return datetime(*(parsed)[0:6])