Seems to me that YAML just needs type/schema support to be less of a hurdle.

As an alternative, the encoding/decoding roundtrip using protobuf seems reasonable to me, catches the footgun of using floating-point version numbers (it becomes a parse error), whitespace/multiline concatenation being more obvious, and allowing comments (compared to JSON):

  ( cat << EOF
  # yes, comments are allowed
  name: "Python package"
  on: "push"
  build {
    python_version: ["3.6", "3.7", "3.8", "3.9", "3.10"]
    steps: [
      {
        name: "Install dependencies"
          run:
            "python -m pip install --upgrade pip\n"
            "pip install pytest\n"
            "if [ -f 'requirements.txt' ]; then pip install -r requirements.txt; fi\n"
      },
      {
        name: "Test with pytest"
        run: "pytest\n"
      }
    ]
  }
  EOF
  ) | protoc --encode=Config config.proto  | protoc --decode=Config config.proto
  
  name: "Python package"
  on: "push"
  build {
    python_version: "3.6"
    python_version: "3.7"
    python_version: "3.8"
    python_version: "3.9"
    python_version: "3.10"
    steps {
      name: "Install dependencies"
      run: "python -m pip install --upgrade pip\npip install pytest\nif [ -f \'requirements.txt\' ]; then pip   install -r requirements.txt; fi\n"
    }
    steps {
      name: "Test with pytest"
      run: "pytest\n"
    }
  }