Add Python up to 3.8 to classifiers

Not adding 3.9 and above as we don't test it yet.

Author: vrusinov

setup.py

@@ -47,6 +47,8 @@
           "Programming Language :: Python :: 3.4",
           "Programming Language :: Python :: 3.5",
           "Programming Language :: Python :: 3.6",
+          "Programming Language :: Python :: 3.7",
+          "Programming Language :: Python :: 3.8",
           "License :: OSI Approved :: GNU General Public License (GPL)",
           "Intended Audience :: Developers",
       ],