Reintroduce sync API.

[tomchristie]

Closes #572

• Added Client.
• Top-level API becomes sync.
• Added URLLib3Dispatcher.
• Wire up timeouts.
• Wire up pool limits.
• Catch and wrap urllib3 networking exceptions.
• Add WSGIDispatcher.
• Add urllib3 proxy dispatcher.

[tomchristie]

Figure we can probably mark this as ready for review here.

We probably still want to do a bit of work on wrapping exception classes, but that could be a follow-on PR.

• The pool limits don't quite match up to what urllib3 uses, but it'll do.
• write_timeout isn't supported.
• I don't think we have any integration tests for proxy support, so I've not verified the proxy case properly yet.
• Not sure that urllib3 supports the CONNECT_ONLY/TUNNEL_ONLY modes.

[florimondmanca]

Yup, I've looked at this for some time and played with it in the console, looks good! 🥇 Just a few minor comments.

Though I can't wait for us unasync'ing all this duplicated code in the client, maybe someday? 🤣

[tomchristie]

Though I can't wait for us unasync'ing all this duplicated code in the client, maybe someday? 🤣

I think that one is a solid maybe, and could go either way. 😃

The only non-trivial chunk of stuff that unasync would deduplicate is https://github.com/encode/httpx/blob/sync/httpx/client.py#L1040-L1216 which is a fairly modest amount for us to maintain, so I could equally well see an unasync approach only ever applied to the dispatch layer. But open to either.

[florimondmanca]

In any case I think we'll start with the dispatch layer, if/when we switch away from urllib3. :-)