gh-149571: Make Element.itertext consistent for Comment and ProcessingInstruction

[hardiksingh5864]

This fixes a behavior mismatch between the pure-Python and C implementations of xml.etree.ElementTree.Element.itertext()
Added regression coverage in Lib/test/test_xml_etree.py

• Issue: Element.itertext() works differently in C and Python implementations #149571

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[StanFromIreland]

It seems you ran a linter, causing a massive diff. Please revert the unrelated changes.

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