Flaky `test_keepalive_conn_management` under Python 3.5 on macOS 10.5

[webknjaz]

❓ I'm submitting a ...

• 🐞 bug report
• 🐣 feature request
• ❓ question about the decisions made in the repository

🐞 Describe the bug. What is the current behavior?

The test occasionally fails in the GitHub Actions workflow.

❓ What is the motivation / use case for changing the behavior?

Stable tests.

💡 To Reproduce

This is observed @ https://github.com/cherrypy/cheroot/runs/323918342#step:13:300

💡 Expected behavior

No failures.

📋 Details

I assume that this may be happening at busy worker VMs so that timeouts are stretched in time.

📋 Environment

• Cheroot version: master
• CherryPy version: N/A
• Python version: 3.5
• OS: macOS 10.15
• Browser: N/A

📋 Additional context

N/A

[webknjaz]

@the-allanc any ideas? It's a test from https://github.com/cherrypy/cheroot/pull/199/files#diff-35273312d4ccd4e42390efe84606cd44R393.