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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _parse_request_range( range_header: str ) -> Optional[Tuple[Optional[int], Optional[int]]]: """Parses a Range header. Returns either ``None`` or tuple ``(start, end)``. Note that while the HTTP headers use inclusive byte positions, this method returns indexes suitable for use in slices. (1, 3) [1, 2] (6, None) (-6, None) (None, 0) (None, None) Note: only supports one range (ex, ``bytes=1-2,6-10`` is not allowed). See [0] for the details of the range header. [0]: http://greenbytes.de/tech/webdav/draft-ietf-httpbis-p5-range-latest.html#byte.ranges """

    unit, _, value = range_header.partition("=")
    unit, value = unit.strip(), value.strip()
    if unit != "bytes":
        return None
    start_b, _, end_b = value.partition("-")
    try:
        start = _int_or_none(start_b)
        end = _int_or_none(end_b)
    except ValueError:
        return None
    if end is not None:
        if start is None:
            if end != 0:
                start = -end
                end = None
        else:
            end += 1
    return (start, end)