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Changes:
- New: clear_cache, Clear django cache, useful when testing or deploying
- Improvement: AutoSlugField, add the possibility to define a custom slugify function
- Improvement: shell_plus --notebook, add a big warning when the notebook extension is not going to be loaded
- Improvement: setup.py, add pypy classifier
- Improvement: readme, add pypy badges
- Fix: admin_generator, Fixed Python 3 __unicode__/__str__ compatibility