add py 3.7 in the list of supported python versions

Author: Lawrence Nahum

setup.py

@@ -22,6 +22,7 @@
         'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
     ],
 
     'keywords': 'Bitcoin wallet BIP32 BIP38 BIP39 secp256k1',