Only apply OCR to files

ahisto_ocr/cli.py

@@ -61,6 +61,7 @@ def main(input_dir: str, output_dir: str, pero_ocr: bool, super_resolution: bool
         filename.relative_to(input_dir)
         for filename
         in input_dir.glob('**/*.*')
+        if filename.is_file()
     )
 
     if not input_images: