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
diff --git a/Lib/test/test_ssl.py b/Lib/test/test_ssl.py
index 67850c3..1710dda 100644
--- a/Lib/test/test_ssl.py
+++ b/Lib/test/test_ssl.py
@@ -1305,6 +1305,8 @@ def test_verify_flags(self):
         self.assertEqual(ctx.verify_flags, ssl.VERIFY_CRL_CHECK_CHAIN)
         ctx.verify_flags = ssl.VERIFY_DEFAULT
         self.assertEqual(ctx.verify_flags, ssl.VERIFY_DEFAULT)
+        ctx.verify_flags = ssl.VERIFY_ALLOW_PROXY_CERTS
+        self.assertEqual(ctx.verify_flags, ssl.VERIFY_ALLOW_PROXY_CERTS)
         # supports any value
         ctx.verify_flags = ssl.VERIFY_CRL_CHECK_LEAF | ssl.VERIFY_X509_STRICT
         self.assertEqual(ctx.verify_flags,