Rename license file

To match conventions using uppercase name

Issue #5 (#5)

setup.py

@@ -124,7 +124,7 @@ setup(
         'Topic :: Software Development :: Libraries :: Python Module',
 
         # Pick your license as you wish
-        'License :: OSI Approved :: GNU General Public License v3 (GPLv3)',
+        'License :: OSI Approved :: GNU General Public License v3 or later (GPLv3+)',
 
         # Specify the Python versions you support here. In particular, ensure
         # that you indicate whether you support Python 2, Python 3 or both.