gh-130947: Add again PySequence_Fast() to the limited C API

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Add again PySequence_Fast(), PySequence_Fast_GET_SIZE() and PySequence_Fast_GET_ITEM() to the limited C API

Add an unit tests.

• Issue: PyQt6 uses PySequence_Fast() which was removed from the limited C API 3.14 #130947

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📚 Documentation preview 📚: https://cpython-previews--130948.org.readthedocs.build/

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This PR basically does the opposite of PR #129398 which is part of Python v3.14.0a5 release. Instead of removing these APIs from the limited C API, this PR fixes the limited C API implementation of PySequence_Fast_GET_SIZE() and PySequence_Fast_GET_ITEM() macros, and it adds again PySequence_Fast() to the limited C API.

PySequence_Fast_ITEMS() remains removed, since this function should be implemented in the limited C API.

What changed since PR #129398 is that I discovered (via a bug report) that PyQt6 uses PySequence_Fast(). Moreover PyQt6 reimplements PySequence_Fast_GET_SIZE() and PySequence_Fast_GET_ITEM() macros in the limited C API.

cc @serhiy-storchaka @encukou

[serhiy-storchaka]

This should be considered a new feature and discussed with the C API Workgroup. Also, I do not think that it is right to replace fast macros with slower functions.

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Also, I do not think that it is right to replace fast macros with slower functions.

Fast macros are not replaced with slower functions. In the regular C API, this PR changes nothing. It only changes the limited C API implementation to use function calls.