partial: Replace returns plugin with an annotation.

The returns plugin hasn’t been updated for mypy ≥ 1.6.  This
annotation is more limited in that it only supports a fixed number of
positional arguments and no keyword arguments, but is good enough for
our purposes.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

tools/semgrep-py.yml

@@ -318,7 +318,7 @@ rules:
 
   - id: functools-partial
     pattern: functools.partial
-    message: "Replace functools.partial with returns.curry.partial for type safety"
+    message: "Replace functools.partial with zerver.lib.partial.partial for type safety"
     languages: [python]
     severity: ERROR