`!` pattern not working with `ignore:`

[dabrahams]

Describe the bug

ignore:
  - !Sources/

in codecov.yml produces:

yaml.constructor.ConstructorError: could not determine a constructor for the tag '!Sources/'
  in "<unicode string>", line 2, column 5:
      - !Sources/
        ^

Environment (please complete the following information):

• System: GitHub CI.

To Reproduce

See https://github.com/hylo-lang/Swifty-LLVM/actions/runs/9132968118/job/25115449559?pr=73#step:17:56

You can clone the project and run it yourself if you want to reproduce.

Expected behavior All paths other than those beginning with Sources/ are ignored.