Add missing AnyIO import

[EmmanuelNiyonshuti]

Summary

httpcore/_backends/anyio improperly uses the anyio.abc module without importing it. With AnyIO 4.15.0's lazy re-exports, much fewer modules are imported as a side-effect which then exposed this problem.

This PR add missing AnyIO import used in httpcore/_backends/anyio.
Ref: agronholm/anyio#1308

Checklist

• [] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [] I've updated the documentation accordingly.

[EmmanuelNiyonshuti]

I am not sure the ci failure is related to this PR.

[agronholm]

This PR is not likely to be ever merged, as httpcore is no longer used by any maintained downstream project.

[lovelydinosaur]

This particular issues looks like it’ll merit a new httpx release. Presumably that changed has broken a large number of async installs.

[agronholm]

I do have a workaround PR almost ready.

[EmmanuelNiyonshuti]

AnyIO 4.15.1 includes a workaround for this. It's resolved regardless of whether this PR get merged. I'll go ahead and close this.

[agronholm]

It's a trivial change and improves correctness so I'd vote for it being merged anyway.