Update the license classifier to "Unlicense" (#180)

Fixes #179

setup.cfg

@@ -11,7 +11,7 @@ license_file = LICENSE
 classifiers =
     Development Status :: 5 - Production/Stable
     Intended Audience :: Developers
-    License :: Public Domain
+    License :: OSI Approved :: The Unlicense (Unlicense)
     Operating System :: OS Independent
     Programming Language :: Python
     Programming Language :: Python :: 3