add Python 3.8 classifier to setup.py

Author: basnijholt

setup.py

@@ -66,6 +66,7 @@ def get_version_and_cmdclass(package_name):
         "Intended Audience :: Science/Research",
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
+        "Programming Language :: Python :: 3.8",
     ],
     packages=find_packages("."),
     install_requires=install_requires,