Introduce new SSLContext API & escalate deprecations.

[lovelydinosaur]

Branch for Version 1.0, so that we can incrementally merge ongoing changes and figure out at what point we want to cut a release.

• Add release notes.
  • Why choose HTTPX.
    • Sync + async support.
    • HTTP/1.1 + HTTP/2 support.
    • Command line client.
    • Strict typing.
    • Timeouts by default.
    • Usage as WSGI/ASGI test client.
    • Well defined transport API.
    • URL, Headers, QueryParams models.
    • Low-level API.
    • Roadmap for future improvements.
  • Document version changes.
  • Sponsorship callout. (Related... I need to conduct an end-of-month appraisal)
• Increment version.
• Merge changes.
  • Improved SSL configuration API.
  • Error when conflicting transport options passed.
  • Reduced dependency footprint.
  • Added NetworkOptions.
  • Deprecation for proxies escalated into removal.
  • Deprecation for app escalated into removal.
  • Removed undocumented raw_url interface.
  • Removed undocumented unnamed 4-tuple timeout.

---

Possible roadmap items...

• Response timeouts.
• Response max size.
• Added routing.
• Stricter typing for auth, proxy & timeout.
• Stricter typing for default_encoding.
• Stricter typing for params, headers & cookies.
• Stricter typing for content, data, files.
• Improved exception messaging for different network failure cases.

...

• DNS caching.
• Expose urlparse & friends.
• Add URLLib3Transport.
• Add AIOHttpTransport.
• Built-in WebSocket support.
• HTTP/3 support.

[karpetrosyan]

Add URLLib3Transport.

I have updated the urllib3 transport from the gist, and it seems ready to use as well as a good example of how transport can be implemented.

I've also worked a bit on AiohttpTransport, and hopefully, we'll see it soon. We could emphasize the possibility of using httpx as a Sans-IO library, capable of working with HTTP streams.

[RobertCraigie]

Would be very excited to see built-in websocket support on the roadmap!

[lovelydinosaur]

I'm reviewing this and thinking we ought to be aiming for an interim 0.28 release here...

• Introduce @karpetrosyan's work on improving the ssl_context = ... API.
• Continue to include deprecatedcert/verify` parameters, so we can get this merged and released.

[lovelydinosaur]

Since we're now deprecating rather than removing verify and cert, I'd like to merge this into mainline now.
We can take a call later on if we'll issue any interim 0.28+ releases, or if we'll move directly to 1.0.