We need to change the setup.py to indicate the library is python 3.5+ only, and do a pass for any bytes/str checks in the code that should be simplified to the python 3 variant only.

We'll probably pin this to 0.4 release rather than doing a major release since this project is still a beta project without a 1.0 release yet.

Currently, nbcelltests' tests use various .ipynb files stored in nbcelltests/tests. Instead, we should generate notebooks as needed during the tests.

Doing this will allow various bits of test code to be simplified.