Use universal newlines in subprocess output to prevent tests

  dulwich.tests.test_blackbox.GitReceivePackTests:test_missing_arg
  dulwich.tests.test_blackbox.GitUploadPackTests:test_missing_arg

to fail because of mismatched line terminators on Windows.

dulwich/tests/__init__.py

@@ -68,6 +68,7 @@ class BlackboxTestCase(TestCase):
         return subprocess.Popen(argv,
             stdout=subprocess.PIPE,
             stdin=subprocess.PIPE, stderr=subprocess.PIPE,
+            universal_newlines=True,
             env=env)