Compatibility with requests

Although we try our best to mimic the requests API, some functionality is not easy to implement and left out. Here are a list of known incompatibilities:

• files API are slightly different, but more error-proof.

• retries are not supported yet, tracked in [#24](https://github.com/yifeikong/curl_cffi/issues/24)

• redirect history are not supported, tracked in [#82](https://github.com/yifeikong/curl_cffi/issues/82)

• empty-domains cookies may lost during redirects, tracked in [#55](https://github.com/yifeikong/curl_cffi/issues/55)