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/_sqlite/cursor.c

@@ -266,7 +266,7 @@ _pysqlite_fetch_one_row(pysqlite_Cursor* self)
                 item = PyBytes_FromStringAndSize(val_str, nbytes);
                 if (!item)
                     goto error;
-                converted = _PyObject_CallOneArg(converter, item);
+                converted = PyObject_CallOneArg(converter, item);
                 Py_DECREF(item);
             }
         } else {