Expose the requests interface to the top package

[lexiforest]

As we have observed how people are using curl_cffi, it seems that curl_cffi almost always co-exists with requests, thus making the intended way of from curl_cffi import requests less useful. A lot of people are using something like this:

from curl_cffi import requests as curl_requests

curl_requests.get(url, ...)

After this change, users can do this

import curl_cffi

curl_cffi.get(url, ...)

Much fewer keystrokes!