Issue 34670: Add set_post_handshake_auth for TLS 1.3

TLS 1.3 removed renegotiation in favor of rekeying and post handshake authentication (PHA). With PHA, a server can request a client certificate from a client at some point after the handshake. The feature is commonly used by HTTP server for conditional and path specific TLS client auth. For example a server can decide to require a cert based on HTTP method and/or path. A client must announce support for PHA during the handshake

Apache mod_ssl uses PHA, https://github.com/apache/httpd/blob/trunk/modules/ssl/ssl_engine_kernel.c#L1207

As of OpenSSL ticket https://github.com/openssl/openssl/issues/6933, TLS 1.3 clients no longer send the PHA TLS extension by default. Nikos and I requested the change, because PHA breaks some assumptions of TLS 1.2 clients. For on-demand auth, PHA extension must be enabled with SSL_CTX_set_post_handshake_auth(), https://www.openssl.org/docs/manmaster/man3/SSL_CTX_set_post_handshake_auth.html .

I propose to add a property on SSLContext to enable PHA and backport the change to Python 2.7, 3.6 and 3.7.

In order to test the feature, I'd also have to add some flags and a function for the server side: (SSL_VERIFY_CLIENT_ONCE, SSL_VERIFY_POST_HANDSHAKE, SSL_verify_client_post_handshake()).

Please note that SSL_verify_client_post_handshake() doesn't perform any IO by itself.

A typical scenario for HTTP looks like this (actual flow may vary):

* client
  * send ``HTTP GET /path``
* server
  * recv
  * verify_client_post_handshake
  * send HTTP Connection Upgrade (emits CertRequest message)
* client
  * recv
  * send upgrade confirmation (emits Certificate, CertificateVerify, Finish message)
* server
  * recv
  * verify certificate
  * send payload or error (may emit TLS alert for unknown, invalid, or missing cert)
* client
  * recv (receive TLS alert or server response)

New changeset 9fb051f032c36b9f6086b79086b4d6b7755a3d70 by Miss Islington (bot) (Christian Heimes) in branch 'master':
bpo-34670: Add TLS 1.3 post handshake auth (GH-9460)
https://github.com/python/cpython/commit/9fb051f032c36b9f6086b79086b4d6b7755a3d70

New changeset 2756ef31656399a120589b7aa19c32e2b91a4758 by Miss Islington (bot) (Christian Heimes) in branch '3.7':
[3.7] bpo-34670: Add TLS 1.3 post handshake auth (GH-9460) (GH-9505)
https://github.com/python/cpython/commit/2756ef31656399a120589b7aa19c32e2b91a4758

New changeset 94812f717dde8b11a9ad9c0fd5be66ff9bd53f58 by Miss Islington (bot) (Christian Heimes) in branch '3.6':
[3.6] bpo-34670: Add TLS 1.3 post handshake auth (GH-9460) (GH-9507)
https://github.com/python/cpython/commit/94812f717dde8b11a9ad9c0fd5be66ff9bd53f58

Will this be backported to the 2.7 branch as well? Pretty please =)

FYI Christian, your "typical scenario for HTTP" doesn't make sense to me... you can't send HTTP Connection Upgrade in the middle of a regular request/response cycle. I feel like the typical scenario ought to be more like:

* client
  * send ``HTTP GET /path``
* server
  * recv
  * verify_client_post_handshake (maybe... via calling SSL_do_handshake again?)
* client
  * recv
  * send upgrade confirmation (emits Certificate, CertificateVerify, Finish message)
* server
  * recv
  * verify certificate
  * send either the requested response, or a 401 Unauthorized depending

But I don't really understand the underlying design here, either at the TLS 1.3 level or the openssl level, and haven't found very useful docs yet, so I could be wrong.