Parse an Upgrade protocol from ``header`` at the given position.

    Return the protocol value and the new position.

    Raise :exc:`~websockets.exceptions.InvalidHeaderFormat` on invalid inputs.

def parse_upgrade_protocol(
    header: str, pos: int, header_name: str
) -> Tuple[UpgradeProtocol, int]:
    """
    Parse an Upgrade protocol from ``header`` at the given position.

    Return the protocol value and the new position.

    Raise :exc:`~websockets.exceptions.InvalidHeaderFormat` on invalid inputs.

    """
    match = _protocol_re.match(header, pos)
    if match is None:
        raise InvalidHeaderFormat(header_name, "expected protocol", header, pos)
    return cast(UpgradeProtocol, match.group()), match.end()