Failed conversion from ONNX to IR

[vgavrilo]

Root cause analysis: According to https://phabricator.wikimedia.org/T213814, the method xml.etree.ElementTree.Element.getchildren() is deprecated from Python 3.2 and has been removed in Python 3.9. As a result, we have failure of conversion of some ONNX models, if the Python version is greater or equal than 3.9.

Solution: Replace element.getchildren() with list(element) in the file model-optimizer\mo\back\ie_ir_ver_2\emitter.py.

Ticket: 55705

Code:

• Comments
• Code style (PEP8)
• Transformation generates reshape-able IR: N/A because there are no new transformations
• Transformation preserves original framework node names: N/A because there are no new transformations

Validation:

• Unit tests: N/A
• Framework operation tests: N/A - No new enabled operations;
• Transformation tests: N/A - No new implemented transformations;
• Model Optimizer IR Reader check: N/A

Documentation:

• Supported frameworks operations list: N/A - No new supported operations;
• Supported public models list: N/A - Nothing to add;
• User guide update: N/A - Nothing to update.