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

Modules/_csv.c

@@ -514,10 +514,10 @@ _call_dialect(PyObject *dialect_inst, PyObject *kwargs)
 {
     PyObject *type = (PyObject *)&Dialect_Type;
     if (dialect_inst) {
-        return _PyObject_FastCallDict(type, &dialect_inst, 1, kwargs);
+        return PyObject_VectorcallDict(type, &dialect_inst, 1, kwargs);
     }
     else {
-        return _PyObject_FastCallDict(type, NULL, 0, kwargs);
+        return PyObject_VectorcallDict(type, NULL, 0, kwargs);
     }
 }
 
@@ -1240,7 +1240,7 @@ csv_writerow(WriterObj *self, PyObject *seq)
     if (line == NULL) {
         return NULL;
     }
-    result = _PyObject_CallOneArg(self->write, line);
+    result = PyObject_CallOneArg(self->write, line);
     Py_DECREF(line);
     return result;
 }