Make the GPL exception explicit in setup.py

I think that is best to have that explicitly set there (and it should possibly be updated in Pypi too).
Thanks for this lib!
Cordially
Philippe

setup.py

@@ -184,7 +184,7 @@ setup(name='pygit2',
       version=__version__,
       url='http://github.com/libgit2/pygit2',
       classifiers=classifiers,
-      license='GPLv2',
+      license='GPLv2 with linking exception',
       maintainer=u('J. David Ibáñez'),
       maintainer_email='jdavid.ibp@gmail.com',
       long_description=long_description,