bpo-44022: Fix http client infinite line reading (DoS) after a HTTP 100 Continue (GH-25916) (GH-25931)

Fixes http.client potential denial of service where it could get stuck reading lines from a malicious server after a 100 Continue response.

Co-authored-by: Gregory P. Smith <greg@krypto.org>
(cherry picked from commit 47895e31b6f626bc6ce47d175fe9d43c1098909d)

Co-authored-by: Gen Xu <xgbarry@gmail.com>