[Python3 migration]Fix test failure in test_sub_port_l2_forwarding.py

Summary:
Fixes sonic-net#7884 [test_sub_port_l2_forwarding] [test issue] TypeError: a bytes-like object is required, not 'str'

What is the motivation for this PR?
test sub_port_interfaces/test_sub_port_l2_forwarding.py fail over python3 issue:
TypeError: a bytes-like object is required, not 'str'

How did you do it?
In Python3, a string cannot work with bytes
Use str to convert bytes to string

How did you verify/test it?
The change can work both in Python2 and Python3.

tests/sub_port_interfaces/test_sub_port_l2_forwarding.py

@@ -110,9 +110,9 @@ def check_no_cpu_packets(duthost, port, packet_fingerprint):
     def verify_no_packet_received(ptfadapter, ports, packet_fingerprint):
         for port in ports:
             for packet, _ in ptfadapter.dataplane.packet_queues[(0, port)]:
-                if packet_fingerprint in packet:
+                if packet_fingerprint in str(packet):
                     logging.error("Received packet with fingerprint '%s' on port %s: %s\n", port, packet_fingerprint,
-                                  packet)
+                                  str(packet))
                     pytest.fail("Received packet on port %s" % port)
 
     duthost = duthosts[rand_one_dut_hostname]