Fix issue with invalid escape sequence '\|'.

[aarpon]

When importing nd2eader (with Python 3.12), the following warning is shown:

nd2reader/label_map.py:75: SyntaxWarning: invalid escape sequence '\|'
  regex = re.compile(six.b("""ImageDataSeq\|(\d+)!"""))

Switching to a raw string fixes the issue:

regex = re.compile(six.b(r"ImageDataSeq\|(\d+)!"))