Note the CI dulwich install in the Git backend test docstring

The docstring said these tests require dulwich without saying where that
comes from. Point at the CI install now that one exists.

netbox/core/tests/test_data_backends.py

@@ -62,7 +62,8 @@ class GitBackendCredentialIntegrationTestCase(TestCase):
     Integration tests that verify GitBackend correctly applies credential logic.
 
     These tests require dulwich to be installed and verify the full integration
-    of the credential handling in GitBackend.fetch().
+    of the credential handling in GitBackend.fetch(). dulwich is the optional
+    'git' extra, so CI installs it explicitly; see .github/workflows/ci.yml.
     """
 
     def _get_clone_kwargs(self, url, **params):