change default SimpleQueue to _PySimpleQueue for queue
change default SimpleQueue to _PySimpleQueue for queue

py3.7 deadlock with monkey patch of stdlib thread modules + use of
ThreadPoolExecutor

3.7, ThreadPoolExecuter was changed to use queue.SimpleQueue; on 3.6 it
uses queue.Queue. No issue in 3.6

The python37 default SimpleQueue was implemented with C code, which will
cause hang the main thread.

Commit 2a71b8e introduce eventlet for unittest, but I did not find
why we need it.
Check other project they also use ThreadPoolExecutor, but not all
of them introduce eventlet.

Maybe GreenThreadPoolExecutor is another solution.

Ref:
ThreadPoolExecutor
https://docs.python.org/3.7/library/concurrent.futures.html#threadpoolexecutor

Same isses about ThreadPoolExecutor and eventlet
https://bugs.launchpad.net/designate/+bug/1782647
https://github.com/eventlet/eventlet/issues/508
https://github.com/gevent/gevent/pull/1253
https://github.com/ClericPy/torequests/issues/10
https://bugs.python.org/issue34173
https://github.com/gevent/gevent/issues/1248
https://github.com/gevent/gevent/issues/1251
http://lists.openstack.org/pipermail/openstack-dev/2018-July/132473.html
https://www.gitmemory.com/issue/eventlet/eventlet/508/511101004

Change-Id: I263f7222119d8d0c89cfc2a4758fe376ce1afd60

openstack/nomad