Add support for TLS 1.3 Early Data

[Lukasa]

Deferred: we can't do this until cryptography supports OpenSSL 1.1.1.

It would be very helpful for applications like Twisted to have access to TLS 1.3 Early Data (see this Twisted ticket). To do that PyOpenSSL needs to expose a few functions:

• SSL_SESSION_get_max_early_data
• SSL_write_early_data
• SSL_read_early_data
• SSL_CTX_set_max_early_data

It'd be good to add this support to PyOpenSSL when it becomes possible.

[maaren]

Is there any progress on this?

[reaperhulk]

The next release of pyca/cryptography will be linked against OpenSSL 1.1.1. I'll go ahead and add the bindings for cryptography, but adding the feature itself in pyOpenSSL is dependent upon someone else stepping up.