papi: fix ubuntu 1804 make test socket.close error

papi: fix ubuntu 1804 make test socket.close error

make test failed on ubuntu 18.04
  File "/vpp/src/vpp-api/python/vpp_papi/vpp_stats.py", line 135, in connect
    socket.close(mfd)
AttributeError: module 'socket' has no attribute 'close'

Due to ubuntu 18.04 using python3.6 and
socket.close() is introduced since python3.7

Using os.close to replace socket.close as mfd is a memory fd

Type: fix
Signed-off-by: Tianyu Li <tianyu.li@arm.com>
Change-Id: I6d980fc87ae6c77bbed416879f9b2fcd0a0abe6a