Automatically include LICENSE files in source distribution

[asottile]

In the same way that README files are automatically grafted into the source distribution, it would be nice if LICENSE / COPYING files were also included by default

I see #357 as a related issue, though I'd prefer a solution which doesn't require me to run around to every repository and apply (imo silly) changes like this and this

If this sounds good, I can hack on this, I think it's nothing more than extending add_defaults inside class sdist to perform a similar operation as _add_defaults_standards (but of course make the license file optional)

[asottile]

Seems the changes in 40.8 ~mostly satisfy this (though I'd prefer license_file defaulting to LICENSE if it existed, but I can live with the current implementation)