Merge #13963: tests: Replace usage of tostring() with tobytes()

8845c8aea6 tests: Replace usage of tostring() with tobytes() (Carl Dong)

Pull request description:

  tostring() is deprecated as of python 3.7 and results in stderr output
  causing tests to fail

Tree-SHA512: 8c5bbd6c6127490922add98543ee7719d19e11200e081784adef2f026ddf90d7735da7d0fb41fa4307d0d3450a27e126752c2b01cbd79b0c8a695855aed080ac

test/functional/test_framework/netutil.py

@@ -107,7 +107,7 @@ def all_interfaces():
             max_possible *= 2
         else:
             break
-    namestr = names.tostring()
+    namestr = names.tobytes()
     return [(namestr[i:i+16].split(b'\0', 1)[0],
              socket.inet_ntoa(namestr[i+20:i+24]))
             for i in range(0, outbytes, struct_size)]