Fixup: annotate '_template_basename' method with the correct return t…

…ype, and add a missing 'return' statement

sphinx/util/fileutil.py

@@ -5,7 +5,7 @@
 import os
 import posixpath
 import warnings
-from typing import TYPE_CHECKING, Callable
+from typing import TYPE_CHECKING, Callable, Optional
 
 from docutils.utils import relative_path
 
@@ -17,7 +17,7 @@
     from sphinx.util.template import BaseRenderer
 
 
-def _template_basename(filename: str) -> bool:
+def _template_basename(filename: str) -> Optional[str]:
     # TODO: remove this method
     if filename.lower().endswith('_t'):
         warnings.warn(
@@ -28,6 +28,7 @@ def _template_basename(filename: str) -> bool:
         return filename[:-2]
     elif filename.lower().endswith(".jinja2"):
         return filename[:-7]
+    return None
 
 
 def copy_asset_file(source: str, destination: str,