Flaky `test_popen_timeout`

This is unexpected, I tried pretty hard to make sure this test wouldn't be flaky.
```
______________________________ test_popen_timeout ______________________________
capsys = <_pytest.capture.CaptureFixture object at 0x7fe03fd0bee0>
deftest_popen_timeout(capsys):
> with pytest.raises(subprocess.TimeoutExpired):
E Failed: DID NOT RAISE
distributed/tests/test_utils_test.py:837: Failed
----------------------------- Captured stdout call -----------------------------
------ stdout: returncode -2, ['/usr/share/miniconda3/envs/dask-distributed/bin/python', '-c', '\nimport signal\nimport sys\nimport time\n\nif sys.platform == "win32":\n signal.signal(signal.SIGBREAK, signal.default_int_handler)\n # ^ Cause `CTRL_BREAK_EVENT` on Windows to raise `KeyboardInterrupt`\n\nprint(\'ready\', flush=True)\nwhile True:\n try:\n time.sleep(0.1)\n print("slept", flush=True)\n except KeyboardInterrupt:\n print("interrupted", flush=True)\n'] ------
Traceback (most recent call last):
File "", line 10, in
KeyboardInterrupt
```
https://github.com/dask/distributed/runs/7429218134?check_suite_focus=true#step:11:1850