Remove Python 2 compatibility code

[patricksanders]

We're no longer compatible with Python 2, so we can remove all of the compatibility workarounds.

For example: https://github.com/Netflix-Skunkworks/aardvark/blob/5926d031b8c9534ed3cadf5f452f610b4cb62d89/aardvark/manage.py#L24-L32

This block can be removed and the code updated to use str and input.