Fix packet generator for classes inheritating from list

SetGen is broken for custom list classes as illustrated by the following
example:

    >>> packet = Ether() / IP()
    >>> packet_list = [packet, packet]
    >>> sendp(packet_list)
    ..
    Sent 2 packets.
    >>> # Now let's do the same with a custom class
    >>> class MyList(list): pass
    ...
    >>> weird_packet_list = MyList(packet_list)
    >>> len(weird_packet_list)
    2
    >>> sendp(weird_packet_list)
    .
    Sent 1 packets.
    >>> # Only one packet is sent instead of two.
    >>> # This is due to SetGen using type() instead of insinstance() to check
    >>> # the nature of the arguments. Indeed:
    >>> type(weird_packet_list)
    <class 'scapy.all.MyList'>
    >>> isinstance(weird_packet_list, list)
    True

--HG--
extra : rebase_source : aeb2b7bd46f3cb4cf0491bbea4a38157e93ecb71