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So, the issue was actually that the `requires_compatibilities` attribute is a string in my code and needs to be a list. :man-facepalming:

BTW - if you run `mypy` or use an IDE tool that does type checking in the background - that should catch issues like this for you automatically, as the Pulumi Python SDKs do have type annotations.

<https://www.pulumi.com/docs/reference/pkg/aws/ecs/taskdefinition/#requires_compatibilities_python>

A few more details on this tooling support in a recent blog post at <https://www.pulumi.com/blog/announcing-python-tooling-improvements/>.