AsyncioBackend: DeprecationWarning for StreamReader/StreamWriter in Py3.8

Python 3.8 builds are filled with warnings about asyncio API changes: https://travis-ci.org/encode/httpx/jobs/574559125

Looks like as as of Python 3.8, asyncio.open_connection() is deprecated in favor of asyncio.connect() — see the asyncio 3.8+ docs.

# < 3.8
reader, writer = await asyncio.open_connection('127.0.0.1', 8888)

# 3.8+
async with asyncio.connect('127.0.0.1', 8888) as stream:
    ...
# or:
stream = await asyncio.connect('127.0.0.1', 8888)

This is not a trivial API change, and I don’t know if there are plans to back-port asyncio.connect() to 3.7 and below.

So at first glance resolving this would need to adapt the .connect() method on AsyncioBackend to use one or the other depending on the Python version we’re running on — and also make Reader/Writer pair (and soon Stream) handle both possibilities ?

Originally raised in: https://github.com/encode/httpx/pull/255#discussion_r315904773

[aeros]

I can confirm that we are removing the deprecation warnings for StreamReader, StreamWriter, StreamReaderProtocol, SubprocessProtocol, and Process in Python 3.8. They might be reinstated for 3.9 depending on what direction is taken with the asyncio streaming API, but they shouldn’t be present in 3.8. This applies to any of the new functions added in https://github.com/python/cpython/commit/23b4b697e5b6cc897696f9c0288c187d2d24bff2 (primary commit being reverted).

https://github.com/python/cpython/pull/16455/files is mostly just awaiting final review from the maintainers of asyncio. If Yury or Andrew have anything to add, they can directly edit the PR or open a new one if needed, but the new streaming API reversion will happen for Python 3.8 regardless of what happens with that PR.

[pquentin]

It looks like asyncio is reverting the new streaming API: https://bugs.python.org/issue38242. Does that means the work from #369 is going to be irrelevant?