Add Trove classifiers to clarify support on PyPI

Author: hugovk

setup.py

@@ -29,6 +29,10 @@
         "License :: OSI Approved :: MIT License",
         "Operating System :: OS Independent",
         "Programming Language :: Python",
+        "Programming Language :: Python :: 3",
+        "Programming Language :: Python :: 3.6",
+        "Programming Language :: Python :: 3.7",
+        "Programming Language :: Python :: 3 :: Only",
         "Topic :: Scientific/Engineering",
     ],
     zip_safe=True,