Fix the error in generating the idle connection list

[zhanghanyun]

Fix the error in generating the idle connection list

[tomchristie]

Thanks. There's a linting failure that needs resolving before we can merge this...

https://github.com/encode/httpcore/actions/runs/9440079124/job/25998938060?pr=920

./scripts/lint should auto-resolve this. I'm curious why we've not picked this up in the test suite. Worth taking a look at if we've any test cases for max_keepalive_connections perhaps?

[tomchristie]

We currently have a test case test_connection_pool_with_no_keepalive_connections_allowed which deals with the max_keepalive_connections=0 case.

Is it worth including test_connection_pool_with_one_keepalive_connection_allowed and test_connection_pool_with_two_keepalive_connections_allowed test cases as part of this PR? Would those test cases validate this change?

[tomchristie]

Okay, let's just take this as-is... it's self-evidently an improvement.
If you could add an entry to the CHANGELOG and then let's get this merged.