Add plone theme classifier. [ci skip]

Author: thomasmassmann

setup.py

@@ -43,6 +43,7 @@
         'Framework :: Plone',
         'Framework :: Plone :: 5.0',
         'Framework :: Plone :: 5.1',
+        'Framework :: Plone :: Theme'
         'Intended Audience :: Developers',
         'License :: OSI Approved :: GNU General Public License v2 (GPLv2)',
         'Programming Language :: Python',