gh-111178: Change Argument Clinic signature for METH_O

[vstinner]

Use "PyObject*" for METH_O functions to fix an undefined behavior.

• Issue: UBSan: Calling a function through pointer to incorrect function type is undefined behavior #111178

[vstinner]

@erlend-aasland: Would you mind to review this change? Focus on the first and third commit, the second one only updates generated code.

This change is a follow-up of PR #128447 (commit 537296c).

Example with list_append() signature:

• Before: list_append(PyListObject *self, PyObject *object)
• After: list_append(PyObject *self, PyObject *object)

which is the correct signature for methods: typedef PyObject *(*PyCFunction)(PyObject *, PyObject *);.

[picnixz]

The manual modifications look correct to me but for the AC part I don't know if there are more assumptions I'm not aware of.