Fix regression in SingleFileHTMLBuilder._get_local_toctree

CHANGES

@@ -7,6 +7,8 @@ Bugs fixed
 * Fixed membership testing (``in``) for the :py:class:`str` interface
   of the asset classes (``_CascadingStyleSheet`` and ``_JavaScript``),
   which several extensions relied upon.
+* Fixed a type error in ``SingleFileHTMLBuilder._get_local_toctree``,
+  ``includehidden`` may be passed as a string or a boolean.
 
 Release 7.2.1 (released Aug 17, 2023)
 =====================================

sphinx/builders/singlehtml.py

@@ -64,10 +64,11 @@ def fix_refuris(self, tree: Node) -> None:
                 refnode['refuri'] = fname + refuri[hashindex:]
 
     def _get_local_toctree(self, docname: str, collapse: bool = True, **kwargs: Any) -> str:
-        if kwargs.get('includehidden', 'false').lower() == 'false':
-            kwargs['includehidden'] = False
-        elif kwargs['includehidden'].lower() == 'true':
-            kwargs['includehidden'] = True
+        if isinstance(includehidden := kwargs.get('includehidden'), str):
+            if includehidden.lower() == 'false':
+                kwargs['includehidden'] = False
+            elif includehidden.lower() == 'true':
+                kwargs['includehidden'] = True
         if kwargs.get('maxdepth') == '':
             kwargs.pop('maxdepth')
         toctree = global_toctree_for_doc(self.env, docname, self, collapse=collapse, **kwargs)