Subprocess.Popen is not followed when the Children flag is on

[DamGouz]

Description

Running a subprocess with Popen causes austin to stop monitoring the children

Steps to Reproduce

import os
import sys
import time

if __name__ == "__main__":
    if len(sys.argv) == 1:
        print(os.getpid())
        subprocess.call(['python', 'test.py', 'popen'])
    elif sys.argv[1] == 'popen':
        subprocess.Popen(['python', 'test.py', 'count'])
    elif sys.argv[1] == 'count':
        for i in range(100):
            print(i)
            time.sleep(0.1)

Expected behavior:
I would expect austin to monitor all 3 calls of the script, up to counting to 100.

Actual behavior:
It stops at Popen:
P55636;T0:5c98;C:\tmp\austin_test\test.py:<module>:9;C:\python\Lib\subprocess.py:call:391;C:\python\Lib\subprocess.py:Popen.wait:1264;C:\python\Lib\subprocess.py:Popen._wait:1588 36

Reproduces how often:
100%

Versions

3.6.0