Add py38 package metadata

Now that we are running the Victoria tests that include a
voting py38, we can now add the Python 3.8 metadata to the
package information to reflect that support.

Change-Id: I853112437e2e9610d421cc48e16f1386a0967788
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

setup.cfg

@@ -16,6 +16,7 @@ classifier =
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3.8
 
 [files]
 packages = validations_libs