Use tox feature "extras" to install extras_require dependencies

Avoids duplicating the list of dependencies. They will now always stay
in sync with the setuptools definition. The feature was added in tox
2.4 (2016-10-12), so added a "minversion" to the configuration.

https://tox.readthedocs.io/en/latest/config.html#conf-extras

> extras
>
> A list of “extras” to be installed with the sdist or develop install.
> For example, extras = testing is equivalent to [testing] in a pip
> install command.