Fix description file.

setup.cfg

@@ -3,7 +3,7 @@ name = dulwich
 author = Jelmer Vernooij
 author_email = jelmer@jelmer.uk
 home-page = https://www.dulwich.io/
-description-file = README.md
+description = file:README.md
 summary = Python Git Library
 classifiers =
     Development Status :: 4 - Beta