[3.13] gh-119452: Fix a potential virtual memory allocation denial of service in http.server (GH-119455) (GH-142130)

The CGI server on Windows could consume the amount of memory specified
in the Content-Length header of the request even if the client does not
send such much data. Now it reads the POST request body by chunks,
so that the memory consumption is proportional to the amount of sent
data.
(cherry picked from commit 29c657a1f231c0908796e0c9ff6967e15ab20d9b)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>