Migrate license metadata to SPDX expression per PEP 639

Replace the deprecated `license = {file = "LICENSE"}` TOML table with a
plain SPDX expression (`license = "GPL-3.0-only"`) and an explicit
`license-files` entry, and drop the now-redundant license trove
classifier.

Fixes deprecation warnings from setuptools >= 77.

Signed-off-by: Nils Le Roux <gilbsgilbert@gmail.com>

Author: gilbsgilbs

pyproject.toml

@@ -10,13 +10,13 @@ authors = [
 description = "A CD-DA ripper prioritising accuracy over speed"
 requires-python = ">=3.6"
 readme = "README.md"
-license = {file = "LICENSE"}
+license = "GPL-3.0-only"
+license-files = ["LICENSE"]
 classifiers = [
     "Environment :: Console",
     "Intended Audience :: End Users/Desktop",
     "Topic :: Multimedia :: Sound/Audio",
     "Programming Language :: Python :: 3",
-    "License :: OSI Approved :: GNU General Public License v3 (GPLv3)",
 ]
 dependencies = [
     "musicbrainzngs",