Warn unreachable for scheduler.py

[fjetter]

Most unreachable messages came from incomplete type annotations that didn't include the possibility that the variable could be None.

For instance

ts: TaskState = self.tasks.get(key)
if ts is None:
    print("mypy detects this as unreachable")

but the annotation itself is redundant and is a leftover from the cythonization

[github-actions]

Unit Test Results

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

       15 files  +       6         15 suites  +6   7h 14m 23s ⏱️ + 3h 44m 12s
  2 879 tests ±       0    2 764 ✔️  -      20    84 💤  -   11  28 ❌ +28  3 🔥 +3 
21 330 runs  +8 626  20 330 ✔️ +8 173  966 💤 +419  31 ❌ +31  3 🔥 +3 

For more details on these failures and errors, see this check.

Results for commit 830d80a. ± Comparison against base commit 4239aed.

[fjetter]

All tests failures appear to related to blocked ports. I hope this will be resolved by #6591