Fix style errors.

dulwich/tests/__init__.py

@@ -134,13 +134,13 @@ def self_test_suite():
 
 
 def tutorial_test_suite():
-    import dulwich.client
-    import dulwich.config
-    import dulwich.index
-    import dulwich.reflog
-    import dulwich.repo
-    import dulwich.server
-    import dulwich.patch
+    import dulwich.client  # noqa: F401
+    import dulwich.config  # noqa: F401
+    import dulwich.index  # noqa: F401
+    import dulwich.reflog  # noqa: F401
+    import dulwich.repo  # noqa: F401
+    import dulwich.server  # noqa: F401
+    import dulwich.patch  # noqa: F401
     tutorial = [
         'introduction',
         'file-format',