Add license_file to setup.cfg metadata
Add license_file to setup.cfg metadata

Without this, the LICENSE file is never included in the built wheels: this makes it harder for users to comply with the license.
With this addition a file LICENSE.txt will be created in the `xxx.dist-info` directory with the content of the `license_file` file, e.g. the top level LICENSE.
Nudies/click,pallets/click,mitsuhiko/click,pgkelley4/click