fix: use correct py.typed location in setup

[niqodea]

What does this PR do?

Use correct py.typed location in setup.py, accounting for the missing src/diffusers part.

When trying to poetry install diffusers, Mypy still complains about missing stubs. In fact, I could not find py.typed in the library after installation. By using the absolute path to py.typed from the repo root, this problem seems fixed.

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[niqodea]

Never mind, it seems like the original repository is already ok when including it via git. Weird behavior from poetry. Sorry for the useless PR.

[niqodea]

For context, this problem is already addressed by #5326. I erroneously thought the code was already included in the latest release 😬