bpo-39245: Switch to public API for Vectorcall (GH-18460)

The bulk of this patch was generated automatically with:

    for name in \
        PyObject_Vectorcall \
        Py_TPFLAGS_HAVE_VECTORCALL \
        PyObject_VectorcallMethod \
        PyVectorcall_Function \
        PyObject_CallOneArg \
        PyObject_CallMethodNoArgs \
        PyObject_CallMethodOneArg \
    ;
    do
        echo $name
        git grep -lwz _$name | xargs -0 sed -i "s/\b_$name\b/$name/g"
    done

    old=_PyObject_FastCallDict
    new=PyObject_VectorcallDict
    git grep -lwz $old | xargs -0 sed -i "s/\b$old\b/$new/g"

and then cleaned up:

- Revert changes to in docs & news
- Revert changes to backcompat defines in headers
- Nudge misaligned comments
diff --git a/Modules/_randommodule.c b/Modules/_randommodule.c
index 1f4bf74..f0fdb03 100644
--- a/Modules/_randommodule.c
+++ b/Modules/_randommodule.c
@@ -287,7 +287,7 @@
         /* Calling int.__abs__() prevents calling arg.__abs__(), which might
            return an invalid value. See issue #31478. */
         args[0] = arg;
-        n = _PyObject_Vectorcall(_randomstate_global->Long___abs__, args, 0,
+        n = PyObject_Vectorcall(_randomstate_global->Long___abs__, args, 0,
                                          NULL);
     }
     else {