<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 async def async_fetch(url: str, **kwargs) -> Selector: """ Do the fetch in an async style. Args: url (str):
 The url of the site. Returns: Selector: allows you to select parts of HTML text using CSS or XPath expressions. """

    kwargs.setdefault('headers', DEFAULT_HEADERS)
    async with aiohttp.ClientSession(**kwargs) as ses:
        async with ses.get(url, **kwargs) as res:
            html = await res.text()
            tree = Selector(text=html)
            return tree