[3.7] bpo-37428: Don't set PHA verify flag on client side (GH-14421) (GH-14493)

[3.7] bpo-37428: Don't set PHA verify flag on client side (GH-14421) (GH-14493)

SSLContext.post_handshake_auth = True no longer sets
SSL_VERIFY_POST_HANDSHAKE verify flag for client connections. Although the
option is documented as ignored for clients, OpenSSL implicitly enables cert
chain validation when the flag is set.

Signed-off-by: Christian Heimes <christian@python.org>
https://bugs.python.org/issue37428
(cherry picked from commit f0f5930ac88482ef896283db5be9b8d508d077db)

Co-authored-by: Christian Heimes <christian@python.org>
https://bugs.python.org/issue37428