fix(mypy): Add type annotation to fix no-any-return error

The sphinx_argparse_neo module is in mypy's ignore_missing_imports,
causing strip_ansi's return type to be treated as Any. Adding explicit
str annotation ensures mypy knows the variable type throughout the
function.

Author: tony

docs/_ext/sphinx_argparse_neo/parser.py

@@ -570,7 +570,7 @@ def _generate_usage(parser: argparse.ArgumentParser) -> str:
     formatter.add_usage(
         parser.usage, parser._actions, parser._mutually_exclusive_groups
     )
-    usage = formatter.format_help().strip()
+    usage: str = formatter.format_help().strip()
 
     # Strip ANSI codes before checking prefix (handles FORCE_COLOR edge case)
     usage = strip_ansi(usage)