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>

pyproject.toml

@@ -1,8 +1,10 @@
 [project]
 name = "mediawiki-dump-tools"
 version = "0.1.1"
-description = "Add your description here"
+description = "Convert MediaWiki XML database dumps into tabular datasets for research"
 readme = "README.md"
+license = "GPL-3.0-or-later"
+license-files = ["COPYING"]
 requires-python = ">=3.11"
 dependencies = [
     "deltas>=0.7.0",