API: fix a possibly unbound error in core.cancel().

[concretevitamin]

Reported by Pylance in vscode.

Related: pyright sky found a ton of errors, a lot of them related to typing errors caused by decorators (pyright is what Pylance uses: https://microsoft.github.io/pyright/#/). The same as my vscode showing tons of warnings/errors. So we can’t integrate in one go into CI.

» pyright sky/core.py                           1 ↵
/Users/zongheng/Dropbox/workspace/riselab/sky-computing/sky/core.py
  /Users/zongheng/Dropbox/workspace/riselab/sky-computing/sky/core.py:167:30 - error: "_Wrapped[(...), Unknown, (*args: Unknown, **kwargs: Unknown), Unknown]" is not iterable
    "__iter__" method not defined (reportGeneralTypeIssues)
  /Users/zongheng/Dropbox/workspace/riselab/sky-computing/sky/core.py:204:32 - error: Expected 0 positional arguments (reportGeneralTypeIssues)
  /Users/zongheng/Dropbox/workspace/riselab/sky-computing/sky/core.py:319:15 - error: "check_features_are_supported" is not a known member of "None" (reportOptionalMemberAccess)
  /Users/zongheng/Dropbox/workspace/riselab/sky-computing/sky/core.py:332:22 - error: Expected 0 positional arguments (reportGeneralTypeIssues)
  /Users/zongheng/Dropbox/workspace/riselab/sky-computing/sky/core.py:364:22 - error: Expected 0 positional arguments (reportGeneralTypeIssues)
  /Users/zongheng/Dropbox/workspace/riselab/sky-computing/sky/core.py:454:15 - error: "check_features_are_supported" is not a known member of "None" (reportOptionalMemberAccess)
  /Users/zongheng/Dropbox/workspace/riselab/sky-computing/sky/core.py:513:60 - error: Expected 0 positional arguments (reportGeneralTypeIssues)
  /Users/zongheng/Dropbox/workspace/riselab/sky-computing/sky/core.py:746:23 - error: No parameter named "refresh" (reportGeneralTypeIssues)
  /Users/zongheng/Dropbox/workspace/riselab/sky-computing/sky/core.py:746:12 - error: Argument missing for parameter "f" (reportGeneralTypeIssues)
  /Users/zongheng/Dropbox/workspace/riselab/sky-computing/sky/core.py:813:9 - error: Expected 0 positional arguments (reportGeneralTypeIssues)
  /Users/zongheng/Dropbox/workspace/riselab/sky-computing/sky/core.py:875:49 - error: Expected 0 positional arguments (reportGeneralTypeIssues)
11 errors, 0 warnings, 0 informations

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
• All smoke tests: pytest tests/test_smoke.py
  • pytest tests/test_smoke.py --managed-spot
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: bash tests/backward_comaptibility_tests.sh

[Michaelvll]

Great catch @concretevitamin! Thanks for fixing it. LGTM.