Use public API to check closed loops

[nightcityblade]

Fixes #98

Replace calls to the private asyncio BaseEventLoop._check_closed method with the public is_closed API while preserving the existing RuntimeError behavior.

Tests:

• Python 3.12 full suite: 2588 passed, 106 skipped, 4 xfailed, 116 subtests passed
• Python 3.13 focused closed-loop tests: 3 passed
• Black 24.1.0 check on all changed files
• git diff --check

[nightcityblade]

I checked the failing matrix jobs against the unrelated #164 run from October 11. The Ubuntu 3.11 and 3.12 jobs fail on the same borrowed CPython SslProtoHandshakeTests.test_check_hostname_requires_server_hostname cleanup warning there, and the PyPy nightly setup jobs likewise cannot find a nightly PyPy 3.9 build. The macOS jobs reported as failures were cancelled while still waiting for runners (runner_name empty, no steps executed). The changed closed-loop checks pass the full local Python 3.12 suite (2,588 tests) and the Python 3.13 focused coverage; all matrix jobs that completed their tests, apart from the two known baseline SSL failures, are green.