Make build-script-helper.py use python3 (#956)

By aiming to update the Node version to 22.17.0 on Swift Docker for
Swift DocC Render [1], we need to specify the version 3 of python, otherwise
it will try to use python 2 which it's not available in Node 22.

Resolves: rdar://156708326

[1] swiftlang/swift-docker#487

Author: marinaaisa

build-script-helper.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """
  This source file is part of the Swift.org open source project
@@ -13,8 +13,6 @@
  knows how to build and install the swift-docc-render.
 """
 
-from __future__ import print_function
-
 import argparse
 import os
 import platform
@@ -75,7 +73,10 @@ def ensure_npm_is_installed(verbose=False):
         fatal_error('-- Error: %s' % error_msg)
     try:
         node_version = check_output(['node', '--version'], verbose=verbose)
-        if not node_version.startswith('v18.16.'):
+        # Ensure node_version is a string (decode if it's bytes)
+        if isinstance(node_version, bytes):
+            node_version = node_version.decode('utf-8')
+        if not node_version.strip().startswith('v18.16.'):
             warn_msg = "Unexpected version of 'node' installed. Swift-DocC-Render requires node 18.16.1. "\
                 "See the README.md file for more information about building Swift-DocC-Render."
             printerr('-- Warning: %s' % warn_msg)