Add "NO CHECKED-IN PROTOBUF GENCODE" to Protobuf Python gencode.

PiperOrigin-RevId: 617652266

Author: protobuf-github-bot

src/google/protobuf/compiler/python/generator.cc

@@ -353,6 +353,9 @@ void Generator::PrintTopBoilerplate() const {
   printer_->Print(
       "# -*- coding: utf-8 -*-\n"
       "# Generated by the protocol buffer compiler.  DO NOT EDIT!\n"
+      "# NO CHECKED-IN PROTOBUF "
+      // Intentional line breaker
+      "GENCODE\n"
       "# source: $filename$\n",
       "filename", file_->name());
   if (opensource_runtime_) {