bpo-33570: TLS 1.3 ciphers for OpenSSL 1.1.1 (GH-6976)

Change TLS 1.3 cipher suite settings for compatibility with OpenSSL
1.1.1-pre6 and newer. OpenSSL 1.1.1 will have TLS 1.3 cipers enabled by
default.

Also update multissltests and Travis config to test with latest OpenSSL.

Signed-off-by: Christian Heimes <christian@python.org>