Support installation on older Python versions

[jaraco]

In pytest-dev/pytest#5272, I encountered an issue with this plugin where it took at least two switches to enable/disable this plugin (one for declaring the plugin, another to enable it by command line), and getting both of those to work in a cross Python environment was not trivial.

I ended up creating a sister package, pytest-black-multipy, which one can declare as a dependency on Python 2.7+ and pass --black to pytest, and the tests will run on all Python versions, invoking black on those that support it.

Perhaps this project would consider adopting that functionality natively so that package maintainers could more readily adopt black/pytest-black for projects that still support older Pythons.

[paddycarey]

I'm not opposed to this. I think it could be useful until Py2 support finally runs out at the end of this year. If you want to submit a PR i'll review it.