Update unit test for new pygithub API

A change to the API caused the test to fail:

```
>       mocker.patch.object(Github, "get_repo", side_effect=github.UnknownObjectException(status=42, data="foo"))
E       TypeError: __init__() missing 1 required positional argument: 'headers'
```

Author: per1234

compilesketches/tests/test_compilesketches.py

@@ -352,7 +352,9 @@ def get_pull(self, number):
     github_api_object.get_repo.assert_called_once_with(full_name_or_id=os.environ["GITHUB_REPOSITORY"])
     github_api_object.get_pull.assert_called_once_with(number=42)  # PR number is hardcoded into test file
 
-    mocker.patch.object(Github, "get_repo", side_effect=github.UnknownObjectException(status=42, data="foo"))
+    mocker.patch.object(Github,
+                        "get_repo",
+                        side_effect=github.UnknownObjectException(status=42, data="foo", headers=None))
     with pytest.raises(expected_exception=SystemExit, match="1"):
         compile_sketches.get_pull_request_base_ref()