Restore license classifier, as PyPi apparently needs it.

Author: Electronic-Mango

pyproject.toml

@@ -11,6 +11,7 @@ classifiers = [
     "Development Status :: 3 - Alpha",
     "Intended Audience :: Developers",
     "Topic :: Software Development :: Build Tools",
+    "License :: OSI Approved :: GNU General Public License v3 (GPLv3)",
     "Programming Language :: Python :: 3",
     "Programming Language :: Python :: 3.11",
     "Programming Language :: Python :: 3.12",