Breaks if directory contains subdirectories

[hexjelly]

Traceback (most recent call last):
  File "/home/roger/.local/bin/synthetic", line 8, in <module>
    sys.exit(main())
  File "/home/roger/.local/lib/python3.10/site-packages/synthetic/__main__.py", line 78, in main
    cmd(**args)
  File "/home/roger/.local/lib/python3.10/site-packages/synthetic/iterdata.py", line 89, in parse_documents
    for name, document_path, ground_truth_path in documents:
  File "/home/roger/.local/lib/python3.10/site-packages/synthetic/iterdata.py", line 40, in iter_documents
    kind = filetype.guess(path)
  File "/home/roger/.local/lib/python3.10/site-packages/filetype/filetype.py", line 28, in guess
    return match(obj) if obj else None
  File "/home/roger/.local/lib/python3.10/site-packages/filetype/match.py", line 29, in match
    buf = get_bytes(obj)
  File "/home/roger/.local/lib/python3.10/site-packages/filetype/utils.py", line 80, in get_bytes
    return get_signature_bytes(obj)
  File "/home/roger/.local/lib/python3.10/site-packages/filetype/utils.py", line 24, in get_signature_bytes
    with open(path, 'rb') as fp:
IsADirectoryError: [Errno 21] Is a directory: 'build/stories'