polishing asyncio Streams API

[1st1]

BPO	38164
Nosy	@asvetlov, @cjrh, @ambv, @1st1, @tirkarthi, @aeros
PRs	python/cpython#16148

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields: ```python assignee = 'https://github.com/asvetlov' closed_at = created_at = labels = ['type-bug', '3.8', 'release-blocker', 'expert-asyncio'] title = 'polishing asyncio Streams API' updated_at = user = 'https://github.com/1st1' ``` bugs.python.org fields: ```python activity = actor = 'aeros' assignee = 'asvetlov' closed = True closed_date = closer = 'aeros' components = ['asyncio'] creation = creator = 'yselivanov' dependencies = [] files = [] hgrepos = [] issue_num = 38164 keywords = ['patch'] message_count = 3.0 messages = ['352372', '353493', '353542'] nosy_count = 6.0 nosy_names = ['asvetlov', 'cjrh', 'lukasz.langa', 'yselivanov', 'xtreak', 'aeros'] pr_nums = ['16148'] priority = 'release blocker' resolution = None stage = 'resolved' status = 'closed' superseder = None type = 'behavior' url = 'https://bugs.python.org/issue38164' versions = ['Python 3.8'] ```

[1st1]

Summary of discussion at https://github.com/python-trio/trio/issues/1208:

• asyncio.Stream.write() will start throwing a DeprecationWarning asking people to add an await if they didn't;

• asyncio.Stream.close() will start throwing a DeprecationWarning asking people to add an await if they didn't;

• asyncio.Stream.drain() & asyncio.Stream.wait_closed() will start throwing a DeprecationWarning telling about a scheduled removal (in Python 3.12) when used on Process.std* streams;

• asyncio.Stream.drain() & asyncio.Stream.wait_closed() will not work at all on Streams created via new 3.8 APIs: connect() & StreamServer.

[aeros]

This should no longer be a release blocker for 3.8 with the reversion of the new asyncio streaming API in python/cpython#60659.

[aeros]

Closed by the new asyncio stream API reversion in python/cpython#60689 and python/cpython#60686.