Add Python 3.7 classifier

Author: nicoddemus

setup.py

@@ -27,6 +27,8 @@ def main():
             'Programming Language :: Python :: 3.4',
             'Programming Language :: Python :: 3.5',
             'Programming Language :: Python :: 3.6',
+            'Programming Language :: Python :: 3.7',
+            'Programming Language :: Python :: Implementation :: CPython',
             'Programming Language :: Python :: Implementation :: PyPy',
             ],
         packages=['execnet', 'execnet.script'],