Fix arguments to setup.py - summary doesn't exist.

setup.py

@@ -90,8 +90,8 @@ setup(name='dulwich',
       author="Jelmer Vernooij",
       author_email="jelmer@jelmer.uk",
       url="https://www.dulwich.io/",
-      description=description,
-      summary="Python Git Library",
+      long_description=description,
+      description="Python Git Library",
       version=dulwich_version_string,
       license='Apachev2 or later or GPLv2',
       project_urls={