Make the pyproject.toml PEP 639 compliant

Building with `python -m build` (same command I have been using all along) started warning me about a deprecated way to specify the license, so I migrated to the alternative the former suggested.

Author: amn

pyproject.toml

@@ -11,15 +11,15 @@ classifiers =   [
     "Development Status :: 5 - Production/Stable",
     "Environment :: Console",
     "Intended Audience :: Developers",
-    "License :: OSI Approved :: GNU General Public License v3 (GPLv3)",
     "Operating System :: POSIX",
     "Programming Language :: Python :: 3",
     "Topic :: Software Development :: Libraries",
     "Topic :: Text Processing"
 ]
 description = "A CSS parsing library"
 dynamic = [ "version" ]
-license = { file = "LICENSE.txt" }
+license = "GPL-3.0-only"
+license-files = [ "LICENSE.txt" ]
 readme = "README.md"
 requires-python = ">=3.11"