In ticket #28022 and in thread https://mail.python.org/pipermail/python-dev/2016-September/146366.html I discussed two new protocols for SSLContext: PROTOCOL_TLS_CLIENT and PROTOCOL_TLS_SERVER. A SSLContext with PROTOCOL_TLS_CLIENT can only create connections to a server but cannot wrap server sockets. We can use it to have better defaults (e.g. cert validation and hostname verification for client side) and to make it impossible to confuse a server context with a client context. In the long run I'm planning to deprecate all but PROTOCOL_TLS_CLIENT and PROTOCOL_TLS_SERVER.

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Finally (and this is the biggest) I like to change how the protocols
work. OpenSSL 1.1.0 has deprecated all version specific protocols. Soon
OpenSSL will only support auto-negotiation (formerly known as
PROTOCOL_SSLv23). My patch #26470 added PROTOCOL_TLS as alias for
PROTOCOL_SSLv23. If the last idea is accepted I will remove PROTOCOL_TLS
again. It hasn't been released yet. Instead I'm going to add
PROTOCOL_TLS_CLIENT and PROTOCOL_TLS_SERVER (see
https://www.openssl.org/docs/manmaster/ssl/SSL_CTX_new.html
TLS_server_method(), TLS_client_method()). PROTOCOL_TLS_CLIENT is like
PROTOCOL_SSLv23 but only supports client-side sockets and
PROTOCOL_TLS_SERVER just server-side sockets. In my experience we can't
have a SSLContext with sensible and secure settings for client and
server at the same time. Hostname checking and cert validation is only
sensible for client-side sockets.
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New changeset 3ea641343244 by Christian Heimes in branch 'default':
Issue #28085: Add PROTOCOL_TLS_CLIENT and PROTOCOL_TLS_SERVER for SSLContext
https://hg.python.org/cpython/rev/3ea641343244

I pushed basic support for PROTOCOL_TLS_CLIENT and PROTOCOL_TLS_SERVER in 3ea641343244. I have another patch that removes PROTOCOL_TLS again and changes create_default_context() to return either a server and client SSLContext. The change is a small possibility to break code that used the create_default_context() in a wrong way.

The issue is resolved. I'll address the other PROTOCOL constants in my upcoming PEP.