fixes issue #53 os.rmdir() fails because of 'Resource busy'

Author: komh

Lib/subprocess.py

@@ -1879,7 +1879,7 @@ def _execute_child(self, args, executable, preexec_fn, close_fds,
 
                 if close_fds:
                   mode |= os.P_2_NOINHERIT
-                if threadsafe:
+                if threadsafe or cwd is not None:
                   mode |= os.P_2_THREADSAFE
                 stdfds = [ p2cread, c2pwrite, errwrite ]