The WSGI_ application handler which returns an iterable
    over the "Hello World!" message.

def hello(environ, start_response):
    '''The WSGI_ application handler which returns an iterable
    over the "Hello World!" message.'''
    if environ['REQUEST_METHOD'] == 'GET':
        data = b'Hello World!\n'
        status = '200 OK'
        response_headers = [
            ('Content-type', 'text/plain'),
            ('Content-Length', str(len(data)))
        ]
        start_response(status, response_headers)
        return iter([data])
    else:
        raise MethodNotAllowed