Fixed #7339 -- Added manual calls to the garbage collector. This is required...

Fixed #7339 -- Added manual calls to the garbage collector. This is required for PyPy and Jython; these implementations don't use reference counting, so you can't rely on __del__ being run immediately after del is called. Thanks to Maciej Fijalkowski (fijal) and Leo Soto.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8004 bcc190cf-cafb-0310-a4f2-bffc1f526a37