Python 2.7 doesn't have a threaded pool implementation, just a multiprocessing
one. We have need of a threaded implementation so add some simple class code
to support this.
(From OE-Core rev: 44ae778fefca5112900b870be7a485360c50bc2e)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>