Error when setting traffic_class on IPv6Header

[zvibazak]

We found an error when setting self.traffic_class on IPv6Header when self.flow_label is greater then 2 bytes.
I added a shift-right by 16 and it fixed.

[majibow]

The fix is correct, although I would have removed the bitwise-and on flow label, since its not necessary.
struct.pack_into('!H', self.raw, 0, 0x6000 | (val << 4) | (self.flow_label >> 16))

Otherwise, I would also add a bitwise-and to traffic class, for safety/clarity.
struct.pack_into('!H', self.raw, 0, 0x6000 | ((val & 0x00FF) << 4) | ((self.flow_label & 0x000FFFFF) >> 16))