[manifest] Explicitly terminate the multiprocessing.Pool

[stephenmcgruer]

Python requires that a Pool object be terminated before it is GC'd
(https://docs.python.org/3/library/multiprocessing.html#multiprocessing.pool.Pool):

Warning: multiprocessing.pool objects have internal resources that need
to be properly managed (like any other resource) by using the pool as a
context manager or by calling close() and terminate() manually. Failure
to do this can lead to the process hanging on finalization.

Note that is not correct to rely on the garbage colletor to destroy the
pool as CPython does not assure that the finalizer of the pool will be
called (see object.__del__() for more information).

Chromium has been seeing hangs in 'wpt manifest' invocations since
moving to Python 3 for WPT, which sounds a lot like 'Failure to do this
can lead to the process hanging on finalization.' So let's
terminate our Pool properly :)