setup.py: document that this project works with py 3.8

Signed-off-by: Mattia Rizzolo <mattia@debian.org>

setup.py

@@ -73,6 +73,7 @@ setup(
         "Programming Language :: Python",
         "Programming Language :: Python :: 3",
         "Programming Language :: Python :: 3.7",
+        "Programming Language :: Python :: 3.8",
         "Topic :: Utilities",
     ],
     # https://packaging.python.org/guides/distributing-packages-using-setuptools/#project-urls