Add lint tox environment
Add a tox environment for linting.
Use requirements file for mypy env.
I filed #303 (closed) for the issue that pylint is not running in the CI, we have collected a large number of pylint issues over the course of it not running so having that in the tox -e lint
would make it difficult to identify the pep8 errors. The linting line should be added back when the lint issues are addressed. I left that commented out for now.