Issue #21015: SSL contexts will now automatically select an elliptic curve for ECDH key exchange on OpenSSL 1.0.2 and later, and otherwise default to "prime256v1".

(should also fix a buildbot failure introduced by #20995)
Issue #20913: improve the SSL security considerations to first advocate using create_default_context().
Closes #20975: make date in the interpreter banner a little more consistent