Fix pserve --reload on Py3 (the simple solution)

This is the simpler fix for `pserve --reload` (so please pull either https://github.com/Pylons/pyramid/pull/1244 or this).

Basically with some project setups the reloader thread is guaranteed to crash on Python 3 as the main thread is importing modules in the other thread.
Merge pull request #1245 from ztane/patch-2

Fix pserve --reload on Py3 (the simple solution)