setup.py: declare that diffoscope is good to go with Python 3.7

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

setup.py

@@ -76,6 +76,7 @@ setup(
         'Programming Language :: Python :: 3',
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
         'Topic :: Utilities',
     ],
 )