setup.py: add Python 3.8 to trove classifiers

Author: blueyed

setup.py

@@ -105,6 +105,7 @@ def run(self):
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
+        'Programming Language :: Python :: 3.8',
         'Programming Language :: Python :: Implementation :: CPython',
         'Programming Language :: Python :: Implementation :: PyPy'
     ],