Make readme cite python3-dev
Make readme cite python3-dev

The old Ubuntu pre-install instructions are mosly fine, but ccbv runs on
Python 3 now (unless you're spelunking into older Django versions) so we
need python3-dev rather than python2-dev

refreshoxford/django-cbv-inspector,refreshoxford/django-cbv-inspector,refreshoxford/django-cbv-inspector,refreshoxford/django-cbv-inspector