Add temporary alias for system_python.bzl at previous location . (#16787)

* Add temporary alias for system_python.bzl at previous location .

This was added to protobuf_deps.bzl in cl/629786458 and no longer needs to be specified directly in WORKSPACE. However, an alias is needed to prevent users from breaking when upgrading from 26.x -> 27.x due to the obsolete load pointing to the wrong path.

This alias will be removed in a future release, tentatively 4.30.x.

PiperOrigin-RevId: 631821637

* Add upb/bazel/system_python.bzl (new alias file) to upb copybara config.

PiperOrigin-RevId: 631918306

bazel/system_python.bzl

@@ -0,0 +1,17 @@
+# Copyright (c) 2009-2021, Google LLC
+# All rights reserved.
+#
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file or at
+# https://developers.google.com/open-source/licenses/bsd
+
+"""Temporary alias to repository rule for using Python 3.x headers from the system."""
+
+load(
+    "//python/dist:system_python.bzl",
+    _system_python = "system_python",
+)
+
+# TODO: Temporary alias. This is deprecated and to be removed in a future
+# release. Users should now get system_python from protobuf_deps.bzl.
+system_python = _system_python