Make repo REUSE compliant

[cornelius]

Following up on #53, it would be nice to make the repo REUSE compliant.

The current license is LGPL-2.0-or-later.

Maybe it would be good to mark the repo as LGPL-2.1-or-later. The license allows that. Then we have the current version of the LGPLv2.

I would not move to LGPLv3 to not introduce any compatibility or other issues and the license still allows users to do it, if they want or need to.

Future incoming contributions would then be LGPL-2.1-or-later.

[dfaure-kdab]

No objection from me.