bpo-40849: Expose X509_V_FLAG_PARTIAL_CHAIN ssl flag (GH-20463)



This short PR exposes an openssl flag that  wasn't exposed. I've also updated to doc to reflect the change. It's heavily inspired by 990fcaac3c428569697f62a80fd95ab4d4b93151.
diff --git a/Doc/library/ssl.rst b/Doc/library/ssl.rst
index c954d9c..b9e5435 100644
--- a/Doc/library/ssl.rst
+++ b/Doc/library/ssl.rst
@@ -650,6 +650,17 @@
 
    .. versionadded:: 3.4.4
 
+.. data:: VERIFY_X509_PARTIAL_CHAIN
+
+   Possible value for :attr:`SSLContext.verify_flags`. It instructs OpenSSL to
+   accept intermediate CAs in the trust store to be treated as trust-anchors,
+   in the same way as the self-signed root CA certificates. This makes it
+   possible to trust certificates issued by an intermediate CA without having
+   to trust its ancestor root CA.
+
+   .. versionadded:: 3.10
+
+
 .. class:: VerifyFlags
 
    :class:`enum.IntFlag` collection of VERIFY_* constants.