Update fgpyo/io/__init__.py

Co-authored-by: Nils Homer <[email protected]>

Author: msto

fgpyo/io/__init__.py

@@ -74,7 +74,7 @@ def assert_path_is_readable(path: Path) -> None:
     """
     # stdin is readable
     if path == Path("/dev/stdin"):
-        return
+        return None
 
     assert path.exists(), f"Cannot read non-existent path: {path}"
     assert path.is_file(), f"Cannot read path becasue it is not a file: {path}"