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

Objects/abstract.c

@@ -179,7 +179,7 @@ PyObject_GetItem(PyObject *o, PyObject *key)
             return NULL;
         }
         if (meth) {
-            result = _PyObject_CallOneArg(meth, key);
+            result = PyObject_CallOneArg(meth, key);
             Py_DECREF(meth);
             return result;
         }
@@ -780,7 +780,7 @@ PyObject_Format(PyObject *obj, PyObject *format_spec)
     }
 
     /* And call it. */
-    result = _PyObject_CallOneArg(meth, format_spec);
+    result = PyObject_CallOneArg(meth, format_spec);
     Py_DECREF(meth);
 
     if (result && !PyUnicode_Check(result)) {
@@ -2502,7 +2502,7 @@ object_recursive_isinstance(PyThreadState *tstate, PyObject *inst, PyObject *cls
             return -1;
         }
 
-        PyObject *res = _PyObject_CallOneArg(checker, inst);
+        PyObject *res = PyObject_CallOneArg(checker, inst);
         _Py_LeaveRecursiveCall(tstate);
         Py_DECREF(checker);
 
@@ -2588,7 +2588,7 @@ object_issubclass(PyThreadState *tstate, PyObject *derived, PyObject *cls)
             Py_DECREF(checker);
             return ok;
         }
-        PyObject *res = _PyObject_CallOneArg(checker, derived);
+        PyObject *res = PyObject_CallOneArg(checker, derived);
         _Py_LeaveRecursiveCall(tstate);
         Py_DECREF(checker);
         if (res != NULL) {