bpo-39342: Expose X509_V_FLAG_ALLOW_PROXY_CERTS in ssl module (GH-18011)

Exposes the `X509_V_FLAG_ALLOW_PROXY_CERTS` constant as `ssl.VERIFY_ALLOW_PROXY_CERTS` to allow for proxy certificate validation as described in: https://www.openssl.org/docs/man1.1.1/man7/proxy-certificates.html