Use SPDX license expression in setup.py

Fixes a build-time warning from setuptools:

/tmp/build-env-7cm02e4g/lib/python3.12/site-packages/setuptools/dist.py:765: SetuptoolsDeprecationWarning: License classifiers are deprecated.
!!

        ********************************************************************************
        Please consider removing the following classifiers in favor of a SPDX license expression:

        License :: OSI Approved :: GNU General Public License v3 (GPLv3)

        See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
        ********************************************************************************

!!
  self._finalize_license_expression()

Author: Gedminas, Marius

setup.py

@@ -67,7 +67,7 @@
     author='Marcus Furlong',
     author_email='[email protected]',
     description='Django based patch status monitoring tool for linux systems',
-    license='GPLv3',
+    license='GPL-3.0-or-later',
     keywords='django patch status monitoring linux spacewalk patchman',
     packages=find_packages(),
     install_requires=install_requires,
@@ -84,6 +84,5 @@
         'Environment :: Web Environment',
         'Framework :: Django',
         'Topic :: Internet :: WWW/HTTP :: WSGI :: Application',
-        'License :: OSI Approved :: GNU General Public License v3 (GPLv3)',
     ],
 )