Advertise Python 3.8 support in setup.py (#7602)

Synapse supports Python 3.8. We've been using it in CI for a while now.

changelog.d/7602.doc

@@ -0,0 +1 @@
+Advertise Python 3.8 support in `setup.py`.

setup.py

@@ -114,6 +114,7 @@ setup(
         "Programming Language :: Python :: 3.5",
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
+        "Programming Language :: Python :: 3.8",
     ],
     scripts=["synctl"] + glob.glob("scripts/*"),
     cmdclass={"test": TestCommand},