I have found the solution. The interface is defined via conf.iface. So thats now the correct code:
ip = IPv6(src=localIpv6 , dst = dstIpv6)
tcp = TCP(sport = localPort, dport = port, flags = "S")
raw = Raw(b"x" * size)
packet = ip / tcp / raw
conf.iface = "eth0"
send(packet, verbose = True)