Add signature for Django 3.2 TimestampSigner (#751)

Django 3.2 introduced two new methods: `sign_object` and
`unsign_object` which can sign/unsign "complex data structures" such as
lists, tuples, dictionaries:

https://docs.djangoproject.com/en/3.2/topics/signing/#django.core.signing.TimestampSigner

Because the methods take an arbitrary serializer (a JSON serializer by
default, but not guaranteed), we cannot be sure of the type of `obj`.

django-stubs/core/signing.pyi

@@ -51,3 +51,15 @@ class Signer:
 class TimestampSigner(Signer):
     def timestamp(self) -> str: ...
     def unsign(self, value: str, max_age: Optional[Union[int, timedelta]] = ...) -> str: ...
+    def sign_object(
+        self,
+        obj: Any,
+        serializer: Type[Serializer] = ...,
+        compress: bool = ...,
+    ) -> str: ...
+    def unsign_object(
+        self,
+        signed_obj: str,
+        serializer: Type[Serializer] = ...,
+        max_age: Optional[Union[int, timedelta]] = ...,
+    ) -> Any: ...