Fix test mock

[hugovk]

Fresh upstream test run after 3.13 became security-only:

__________________ test_tag_is_security_release[3.13.3-False] __________________

version = '3.13.3', expected = False
mocker = <pytest_mock.plugin.MockerFixture object at 0x7f8a4d3a2890>

    @pytest.mark.parametrize(
        ["version", "expected"],
        [
            ("3.12.10", True),
            ("3.13.3", False),
        ],
    )
    def test_tag_is_security_release(
        version: str, expected: bool, mocker: MockerFixture
    ) -> None:
        # Arrange
        mock_response = b"""
        {
            "3.13": {
                "status": "bugfix"
            },
            "3.12": {
                "status": "security"
            }
        }
        """
        mocker.patch("urllib.request.urlopen", return_value=io.BytesIO(mock_response))
    
        # Act
        tag = release.Tag(version)
    
        # Assert
>       assert tag.is_security_release is expected
E       assert True is False
E        +  where True = <release.Tag object at 0x7f8a4d2e8d40>.is_security_release

https://github.com/python/release-tools/actions/runs/37047689795/job/110973075475

The mock wasn't properly mocking, and the test was fetching live data.

3.13 just went bugfix->security, so it failed.