Fixed SocketStream close race on asyncio

[subotac]

NOTE Erasing or replacing the contents of this template will result in your pull
request being summarily closed without consideration!

Changes

Fixes #1250.

On the asyncio backend, SocketStream.aclose() closes the transport, yields once, and then aborts it. If the write buffer drains during that checkpoint, connection_lost() may finalize the
transport before abort() runs, causing asyncio to access a transport loop that has already been cleared.

Track whether StreamProtocol.connection_lost() has run and skip the redundant abort in that case. Add a deterministic regression test covering this interleaving.

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) which would fail without your patch
• You've updated the documentation (in docs/), in case of behavior changes or new
  features
  • Not applicable: this fixes an internal cleanup race without changing the documented API.
• You've added a new changelog entry (in docs/versionhistory.rst).

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
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Updating the changelog

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If, say, your patch fixes issue #123, the entry should look like this:

• Fix big bad boo-boo in task groups
  (max_size bs. max_bytes #123 max_size bs. max_bytes #123_; PR by @subotac)

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