ftr/reuse

[dkastl]

Changes proposed in this pull request:

• Introduces REUSE (https://reuse.software/)
• Updates License to GPL-3.0-or-later

@gtt-project/maintainer

[sanak]

@dkastl (CC: @iosefa)
I forgot to mention, but the followings are preferable.

• macOS (Catalina, which I am using) doesn't support Python 3 as default, so adding venv/ to .gitignore.
  (Assume that the following venv installation.)

  brew install python3
  python3 -m venv venv
  source venv/bin/activate
  pip3 install reuse

• GitHub Actions - workflow file also seems to need to have license.

  :
  The following files have no copyright and licensing information:
  * .github/workflows/reuse.yml
  :

[dkastl]

Maybe you could also use it with Docker https://reuse.readthedocs.io/en/stable/readme.html#run-in-docker