Use django.utils.six for PY2/PY3 compatibility

1. Removed file `cms/utils/compat/type_checks.py`, use `django.utils.six` module instead.
1. Use `six.PY3` from `django.utils.six` instead of `PY2` from `cms.utils.compat` to check Python version.
Please add a new entry for 3.0.4 as 3.0.3 was released yesterday