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/_json.c b/Modules/_json.c
index a70043b..ee2070c 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -777,14 +777,14 @@
     *next_idx_ptr = idx + 1;
 
     if (has_pairs_hook) {
-        val = _PyObject_CallOneArg(s->object_pairs_hook, rval);
+        val = PyObject_CallOneArg(s->object_pairs_hook, rval);
         Py_DECREF(rval);
         return val;
     }
 
     /* if object_hook is not None: rval = object_hook(rval) */
     if (s->object_hook != Py_None) {
-        val = _PyObject_CallOneArg(s->object_hook, rval);
+        val = PyObject_CallOneArg(s->object_hook, rval);
         Py_DECREF(rval);
         return val;
     }
@@ -890,7 +890,7 @@
         return NULL;
 
     /* rval = parse_constant(constant) */
-    rval = _PyObject_CallOneArg(s->parse_constant, cstr);
+    rval = PyObject_CallOneArg(s->parse_constant, cstr);
     idx += PyUnicode_GET_LENGTH(cstr);
     Py_DECREF(cstr);
     *next_idx_ptr = idx;
@@ -989,7 +989,7 @@
                                            idx - start);
         if (numstr == NULL)
             return NULL;
-        rval = _PyObject_CallOneArg(custom_func, numstr);
+        rval = PyObject_CallOneArg(custom_func, numstr);
     }
     else {
         Py_ssize_t i, n;
@@ -1399,7 +1399,7 @@
     if (s->fast_encode) {
         return s->fast_encode(NULL, obj);
     }
-    encoded = _PyObject_CallOneArg(s->encoder, obj);
+    encoded = PyObject_CallOneArg(s->encoder, obj);
     if (encoded != NULL && !PyUnicode_Check(encoded)) {
         PyErr_Format(PyExc_TypeError,
                      "encoder() must return a string, not %.80s",
@@ -1485,7 +1485,7 @@
                 return -1;
             }
         }
-        newobj = _PyObject_CallOneArg(s->defaultfn, obj);
+        newobj = PyObject_CallOneArg(s->defaultfn, obj);
         if (newobj == NULL) {
             Py_XDECREF(ident);
             return -1;