In the current implementation of arp_responder, it captures all the packets and then filter the arp packets by the length of packets:

ETH_P_ALL = 0x03
...
self.socket = socket.socket(socket.AF_PACKET, socket.SOCK_RAW, socket.htons(self.ETH_P_ALL))
...
if len(data) != self.ARP_PKT_LEN:

Why not capture ARP packets directly by defining a more accurate packet type? like:

ETH_P_ARP = 0x806
...
self.socket = socket.socket(socket.AF_PACKET, socket.SOCK_RAW, socket.htons(self.ETH_P_ARP))

by this can make arp_responder more efficient and avoid to capture some other type of packets which also have the same length.