use pathlib path in sitemap plugin for sphinx compatibility (ros2#4215)

* use pathlib paths for sphinx compatibility

* conditionally use pathlib if sphinx >=7

plugins/sphinx_sitemap_ros.py

@@ -25,7 +25,9 @@
 from multiprocessing import Manager
 from typing import Any, Dict, List, Optional
 from xml.etree import ElementTree
+from pathlib import Path
 
+import sphinx
 from sphinx.application import Sphinx
 from sphinx.util.logging import getLogger
 
@@ -225,7 +227,11 @@ def create_sitemap(app: Sphinx, exception):
                 href=site_url + scheme.format(lang=lang, version=version, link=link),
             )
 
-    filename = app.outdir + "/" + app.config.sitemap_filename
+    if sphinx.version_info[0] >= 7:
+        filename = Path.joinpath(app.outdir, app.config.sitemap_filename)
+    else:
+        filename = app.outdir + "/" + app.config.sitemap_filename
+
     ElementTree.ElementTree(root).write(
         filename, xml_declaration=True, encoding="utf-8", method="xml"
     )