pass `re.sub` flags as kwargs

[sneakers-the-rat]

hello :)

making some bags in testing, getting a DeprecationWarning:

tests/test_validation.py: 81 warnings
  .venv/bin/bagit.py:1412: DeprecationWarning: 'count' is passed as positional argument
    s = re.sub(r"%0D", "\r", s, re.IGNORECASE)

tests/test_validation.py: 81 warnings
  .venv/bin/bagit.py:1413: DeprecationWarning: 'count' is passed as positional argument
    s = re.sub(r"%0A", "\n", s, re.IGNORECASE)

looks like the flags are being interpreted as count, and we're also being warned that we need to pass extra args as kwargs anyway, so fixed that :)