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/Objects/unicodeobject.c b/Objects/unicodeobject.c
index aa874f2..68e4f6a 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -4250,7 +4250,7 @@
     if (*exceptionObject == NULL)
         goto onError;
 
-    restuple = _PyObject_CallOneArg(*errorHandler, *exceptionObject);
+    restuple = PyObject_CallOneArg(*errorHandler, *exceptionObject);
     if (restuple == NULL)
         goto onError;
     if (!PyTuple_Check(restuple)) {
@@ -4354,7 +4354,7 @@
     if (*exceptionObject == NULL)
         goto onError;
 
-    restuple = _PyObject_CallOneArg(*errorHandler, *exceptionObject);
+    restuple = PyObject_CallOneArg(*errorHandler, *exceptionObject);
     if (restuple == NULL)
         goto onError;
     if (!PyTuple_Check(restuple)) {
@@ -6801,7 +6801,7 @@
     if (*exceptionObject == NULL)
         return NULL;
 
-    restuple = _PyObject_CallOneArg(*errorHandler, *exceptionObject);
+    restuple = PyObject_CallOneArg(*errorHandler, *exceptionObject);
     if (restuple == NULL)
         return NULL;
     if (!PyTuple_Check(restuple)) {
@@ -8783,7 +8783,7 @@
     if (*exceptionObject == NULL)
         return NULL;
 
-    restuple = _PyObject_CallOneArg(*errorHandler, *exceptionObject);
+    restuple = PyObject_CallOneArg(*errorHandler, *exceptionObject);
     if (restuple == NULL)
         return NULL;
     if (!PyTuple_Check(restuple)) {