support/scripts/pkg-stats: clear multiprocessing pools after use

During the CVE checking phase, we can still see a huge amount of
Python processes (actually 128) running on the host, even though
the CVE step is entirely ran in the main thread.

These are actually the worker processes spawned to check for the
packages URL statuses and the latest versions from release-monitoring.
This is because of an issue in Python's multiprocessing implementation:
https://bugs.python.org/issue34172



The problem was already there before the CVE matching step was
introduced, but because pkg-stat was terminating right after the
release-monitoring step, it went unnoticed.

Also, do not hold a reference to the multiprocessing pool from
the Package class, as this is not needed.

Signed-off-by: Titouan Christophe <titouan.christophe@railnova.eu>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>