license under the GNU GPL, version 3 or later

Add the full license text as COPYING (the same text Debian ships in
common-licenses), declare the license and license file in
pyproject.toml, add copyright and permission notices to the source
files, and describe the license in the README. Also replace the
placeholder package description in pyproject.toml.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
(cherry picked from commit 90a14cf999)

README.md

@@ -41,3 +41,10 @@ To run tests:
 1.  \[\] Output metadata about the run. What parameters were used? What
     versions of deltas?
 2.  \[\] Url encoding by default
+
+## License
+
+wikiq is free software: you can redistribute it and/or modify it under
+the terms of the GNU General Public License as published by the Free
+Software Foundation, either version 3 of the License, or (at your
+option) any later version. See [COPYING](COPYING) for the full text.