Parse optional whitespace from ``header`` at the given position.

    Return the new position.

    The whitespace itself isn't returned because it isn't significant.

def parse_OWS(header: str, pos: int) -> int:
    """
    Parse optional whitespace from ``header`` at the given position.

    Return the new position.

    The whitespace itself isn't returned because it isn't significant.

    """
    # There's always a match, possibly empty, whose content doesn't matter.
    match = _OWS_re.match(header, pos)
    assert match is not None
    return match.end()