fix: typing for get_all_duts

[horw]

Description

File "/root/.espressif/python_env/idf5.3_py3.8_env/lib/python3.8/site-packages/pytest_embedded/dut_factory.py", line 854, in DutFactory
def get_all_duts(cls) -> list[Dut]:
TypeError: 'type' object is not subscriptable

Related

Testing

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Checklist

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• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
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