Add fixes to setup.py for the inability to import pip for parsing
requirements files.

Update tox with new coverage commands and only test on a single Python3
version.

Create venv if one doesn't exist with direnv.

Remove encoding pragma as this is no longer necessary as of ~Python3.6?

Remove _compat.py as six does a better job and Python2.7 shouldn't be in
use anymore.

Add the requirements directory text files to the Manifest.in.

Update versions of pre-commit repos.