Add Python 3.8 classifier [skip ci]

Author: Chilipp

setup.py

@@ -26,6 +26,7 @@ def readme():
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
+        'Programming Language :: Python :: 3.8',
         'Operating System :: OS Independent',
       ],
       keywords='sphinx autodoc autosummary content table',