CVE-2023-43804: `Cookie` HTTP header isn't stripped on cross-origin redirects

urllib3 doesn’t treat the Cookie HTTP header special or provide any helpers for managing cookies over HTTP, that is the responsibility of the user. However, it is possible for a user to specify a Cookie header and unknowingly leak information via HTTP redirects to a different origin if that user doesn’t disable redirects explicitly.

Users must handle redirects themselves instead of relying on urllib3’s automatic redirects to achieve safe processing of the Cookie header, thus we decided to strip the header by default in order to further protect users who aren’t using the correct approach.