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I did some investigating using a test script and Python 3.7.0a0
from multiprocessing import Pool
import os
import time
def f(x):
	time.sleep(30)
	return x*x
if __name__=='__main__':
	print('Main pid {0}'.format(os.getpid()))
	p = Pool(5)
	p.map(f, [1,2,3])
	print('Returned')
	time.sleep(30)
and grepping for pipe and the parentpid in the output from lsof ( lsof | grep python.*<parentpid>.*pipe ). The pipes opened at the start of the script are still open even after the line print('Returned') is executed. I suppose this is expected because I did not call *p.close()*. All pipes are cleaned up after the parent process finishes.
When I repeat the experiment calling p.close() after p.map returns, all that is left is the 9 pipes opened by the parent. All pipes are cleaned up after parent script exits. 
@shani - could you please clarify how you were able to detect the leaking pipes?