Add support for Next Protocol Negotiation.

[Lukasa]

This is part of #79.

This is my attempt to remain in the spirit of PyOpenSSL by making a minimal wrapper around OpenSSL's callback logic. As you can see, minimal is not my strong suit. Feedback would be appreciated. =D

Note: this will probably fail automated testing at the moment, as it relies on features that aren't yet in the current cryptography release. Passes on my machine on 2.7.

[exarkun]

Can you add a link to the cryptography pull request that introduces the necessary features? This will make it easier to figure out what release of cryptography this will depend on.

[Lukasa]

Of course, it's pyca/cryptography#857.

[Lukasa]

So the current cryptography master appears to pass (according to Travis) except on 3.2, which fails for reasons unrelated to my change. =)

[myers]

@Lukasa Excited to see NPN to get into pyopenssl.

[alex]

The release has happened, so this should work now.

[Lukasa]

👍 Thanks for your help @alex!

[Lukasa]

@exarkun What can I do to help move this forward now that cryptograpy has a release with the needed bindings? =)

[exarkun]

It might be nice to make Next Protocol Negotiation a link to a document about that concept. Super nice would be a pyOpenSSL-internal doc explaining all the pieces of NPN but considering NPN is kind of obsolete already and there isn't already such a doc, an external link would probably have the best cost/benefit ratio.

[exarkun]

Thanks for your work on this Lukasa. Sorry I didn't get back to you with feedback sooner.

In addition to my inline comments, could you also add documentation for the new methods to doc/api/ssl.rst? Doesn't need to be any different from what you wrote in the docstrings (pyOpenSSL just has two copies of its API docs atm 😢 ).

Also, if you wouldn't mind closing this PR and re-opening it using a branch with a different name than "master" that would be wonderful.

Thanks again.

[Lukasa]

@exarkun No problem. =) There wouldn't have been a great deal to gain before now anyway, since it was all writing against bindings that didn't exist.

I'll handle the markups in the next couple of days. I'm super busy at the minute but this is a high priority for me (I need it for one of my OSS projects), so I won't let this languish for long.

[Lukasa]

@exarkun Ok, I've made the changes you suggested and opened a new pull request from the npn branch. The new PR is #110. Let me know what further changes you want! =)