pyproject.toml: add SPDX license syntax.

Author: Benjamin Moody

pyproject.toml

@@ -9,10 +9,10 @@ authors     = [
 classifiers = [
   "Development Status :: 4 - Beta",
   "Intended Audience :: Developers",
-  "License :: OSI Approved :: GNU General Public License v3 or later (GPLv3+)",
   "Programming Language :: Python :: 3",
   "Topic :: Multimedia :: Sound/Audio :: Conversion",
 ]
+license = "GPL-3.0-or-later"
 requires-python = ">=3.5"
 
 [build-system]