[2.7] bpo-28043: improved default settings for SSLContext (GH-10608)

[2.7] bpo-28043: improved default settings for SSLContext (GH-10608)

The options OP_NO_COMPRESSION, OP_CIPHER_SERVER_PREFERENCE,
OP_SINGLE_DH_USE, OP_SINGLE_ECDH_USE, OP_NO_SSLv2 (except
for PROTOCOL_SSLv2), and OP_NO_SSLv3 (except for PROTOCOL_SSLv3)
are set by default. The initial cipher suite list contains only
HIGH ciphers, no NULL ciphers and MD5 ciphers (except for PROTOCOL_SSLv2).

(cherry picked from commit 358cfd426ccc0fcd6a7940d306602138e76420ae)