setup/PyPI: Add PyPy classifier

Author: akx

setup.py

@@ -51,6 +51,7 @@ def run(self):
         'Programming Language :: Python :: 3.3',
         'Programming Language :: Python :: 3.4',
         'Programming Language :: Python :: 3.5',
+        'Programming Language :: Python :: Implementation :: PyPy',
         'Topic :: Software Development :: Libraries :: Python Modules',
     ],
     packages=['babel', 'babel.messages', 'babel.localtime'],