Optional HTTP/2

[tomchristie]

For consideration.
We might want to think again about having HTTP/2 be an optional install for our httpx 1.0 release onwards.

Done:

• Provide a base HTTP Connection class interface, that is agnostic of HTTP/1.1 and HTTP/2.
• Use the base HTTP Connection class for the .connection signature in AsyncHTTPConnection.
• Lazily import either AsyncHTTP11Connection or AsyncHTTP2Connection as required.

To do:

• Import h2 in AsyncConnectionPool if http2=True is set, so that we get nice early errors if it is not installed.
• Move h2 to an optional install, requiring pip install httpcore[http2]

[tomchristie]

@florimondmanca - I think we ought to look at getting this one in, and treat the remaining issues as a follow up...

• Import h2 in AsyncConnectionPool if http2=True is set, so that we get nice early errors if it is not installed.
• Move h2 to an optional install, requiring pip install httpcore[http2]

That'll get us in good shape and allow us to make a judgement call on if we want optional HTTP/2 for httpx 0.14 or if we're waiting for 1.0.

[florimondmanca]

@tomchristie Okay, I thought you meant to bring the remaining TODOs in this PR which is why I held back on reviewing. Will review soon then. :-)

[florimondmanca]

Very nice!

[tomchristie]

Right, let's do this...