Drop NPN support

[florimondmanca]

@pgjones suggested in #276 (comment) to stop using NPN to get the HTTP protocol in .get_http_version() on concurrency backends, and switch to ALPN-only.

(Technical notes of mine: NPN (Next Protocol Negociation) is a TLS extension that was added as part of SPDY a Google protocol whose spec eventually got merged into the HTTP/2 standard. Google announced this in 2016 in Hello HTTP/2, good bye SPDY, and ALPN now replaces NPN.)

I did a bit of research, and NPN removal does seem to have been on its way across the interwebs:

• Chrome 51, 2016
• Firefox 53, 2017
• Cloudflare, 2018
• Safari, 2019 (macOS Mojave and iOS 12)
• Hypercorn, 2019 (😄)

Anyone more experienced than me on HTTP protocols negociation got any thoughts on this?

[sethmlarson]

You can add macOS to the list as well. The new Network module only supports ALPN.

[lovelydinosaur]

Fair enough, that sounds like a nice little improvement.