* Improve tox.ini by running flake8, mypy, nosetests and sphinx in there own environment.
Improves isolation. Dependencies of one package won't influence the next.
This should fail for the current setup with typing-extensions.
* Try to fix actually running tox tests on travis
* Use recent tox
* Fix using Literal[False] for open_filename.
None has the same true value as False, and therefore it does not matter.
* Replace typing_extensions.Literal by the type of the literal
* Add line to CHANGELOG.md