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/connection.c

@@ -308,7 +308,7 @@ PyObject* pysqlite_connection_cursor(pysqlite_Connection* self, PyObject* args,
         factory = (PyObject*)&pysqlite_CursorType;
     }
 
-    cursor = _PyObject_CallOneArg(factory, (PyObject *)self);
+    cursor = PyObject_CallOneArg(factory, (PyObject *)self);
     if (cursor == NULL)
         return NULL;
     if (!PyObject_TypeCheck(cursor, &pysqlite_CursorType)) {
@@ -975,7 +975,7 @@ static void _trace_callback(void* user_arg, const char* statement_string)
     py_statement = PyUnicode_DecodeUTF8(statement_string,
             strlen(statement_string), "replace");
     if (py_statement) {
-        ret = _PyObject_CallOneArg((PyObject*)user_arg, py_statement);
+        ret = PyObject_CallOneArg((PyObject*)user_arg, py_statement);
         Py_DECREF(py_statement);
     }
 
@@ -1472,7 +1472,7 @@ pysqlite_connection_iterdump(pysqlite_Connection* self, PyObject* args)
         goto finally;
     }
 
-    retval = _PyObject_CallOneArg(pyfn_iterdump, (PyObject *)self);
+    retval = PyObject_CallOneArg(pyfn_iterdump, (PyObject *)self);
 
 finally:
     Py_XDECREF(module);