setup.py: add MIT license to classifiers

[rveachkc]

The project contains an MIT license in the LICENSE file, but this is not
reflected in the package classifiers.

I'd like to consider using red-mail in a project, but the automation requires the license to be listed in the classifiers. Please consider merging.

[Miksus]

Thanks for contributing! Indeed that should be in the classifiers.

I suspect you need a new version published to PyPI in order to pass your automation? If this is the case, I can see if I find time to create a patch release this weekend so you could go on using the package.

[rveachkc]

A patch release would be fantastic, thanks!

[Miksus]

This is now released. Hopefully you are now able to install the package.