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

This is the simpler fix for `pserve --reload` (so please pull either #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.

pyramid/scripts/pserve.py

@@ -811,7 +811,7 @@ def check_reload(self):
                 print(
                     "Error calling reloader callback %r:" % file_callback)
                 traceback.print_exc()
-        for module in sys.modules.values():
+        for module in list(sys.modules.values()):
             try:
                 filename = module.__file__
             except (AttributeError, ImportError):