Fix mypy warning in test_client.py

[crusaderky]

Fix mypy warning:

distributed/tests/test_client.py:4831: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]

The warning appears whenever there's a mypy error anywhere and you run pre-commit run --all-files.

[jrbourbeau]

Thanks @crusaderky -- this LGTM

My guess is the test failures in CI are related to environment caching. I've pushed an empty commit that skips the env cache

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

       26 files  ±0         26 suites  ±0   12h 33m 44s ⏱️ - 1h 6m 12s
  3 542 tests ±0    3 434 ✔️ +154     104 💤 ±0  4 ❌  - 154 
44 788 runs  ±0  42 688 ✔️ +463  2 096 💤 +1  4 ❌  - 464 

For more details on these failures, see this check.

Results for commit a5271f6. ± Comparison against base commit 4b9892e.