[Fixed]-How can I modify the site packages on heroku because of this error 'PeriodicTask' object has no attribute '_default_manager'

I solved this problem by modifying original app on git.
Making clone, modifying it and then adding to requirements.txt this line: git+git://github.com/user/appname@branch_name (you have to use your modified apps url @ branch_name)
This installs your modified app.