Fix invalid GPLv3 trove classifier

"License :: OSI Approved :: GNU General Public License v3" is not a
valid trove classifier; the correct value per the trove-classifiers spec
is "License :: OSI Approved :: GNU General Public License v3 (GPLv3)".

This causes build failures for distribution package maintainers who use
`python -m build --no-isolation`, which exposes the build to a
system-installed validate-pyproject that enforces the classifier spec.
Isolated builds are unaffected.

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

Author: gilbsgilbs

pyproject.toml

@@ -16,7 +16,7 @@ classifiers = [
     "Intended Audience :: End Users/Desktop",
     "Topic :: Multimedia :: Sound/Audio",
     "Programming Language :: Python :: 3",
-    "License :: OSI Approved :: GNU General Public License v3",
+    "License :: OSI Approved :: GNU General Public License v3 (GPLv3)",
 ]
 dependencies = [
     "musicbrainzngs",