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pip_gui.pip_tkinter.utils.runpip has indeed switched to using subprocess (and yes, the PyPA crew are significantly more careful with backwards compatibility there), but there are still a couple of other API imports in that module: https://github.com/upendra-k14/pip_gui/blob/dump_code/pip_tkinter/utils.py#L7 (I didn't check other code to see if the affected APIs are used anywhere)
I do like the idea of *bundling* pip_gui in a variant of the ensurepip style, rather than having it as a conventional standard library component, though - it means we can simplify the backwards compatibility guarantee by noting that, if you're using the provided GUI, doing "pip install --upgrade pip" may also require doing "pip install --upgrade pip-tkinter".