Use os.PathLike over pathlib.Path

Sphinx 7.1 and earlier don't use ``pathlib`` internally.

sphinxcontrib/serializinghtml/__init__.py

@@ -102,7 +102,7 @@ def handle_page(self, pagename: str, ctx: dict, templatename: str = 'page.html',
 
     def handle_finish(self) -> None:
         # dump the global context
-        outfilename = self.outdir / self.globalcontext_filename
+        outfilename = path.join(self.outdir, self.globalcontext_filename)
         self.dump_context(self.globalcontext, outfilename)
 
         # super here to dump the search index