Switch to PEP 639 license format.

pyproject.toml

@@ -5,13 +5,14 @@ authors = [
   { name="Dan Helfman", email="witten@torsion.org" },
 ]
 description = "Simple, configuration-driven backup software for servers and workstations"
+license = "GPL-3.0-or-later"
+license-files = ["LICENSE"]
 readme = "README.md"
 requires-python = ">=3.9"
 classifiers=[
     "Development Status :: 5 - Production/Stable",
     "Environment :: Console",
     "Intended Audience :: System Administrators",
-    "License :: OSI Approved :: GNU General Public License v3 (GPLv3)",
     "Programming Language :: Python",
     "Topic :: Security :: Cryptography",
     "Topic :: System :: Archiving :: Backup",