Allow non-string values in dummy template render context

The implementation of the `render()` method uses the standard library
method `string.Template.safe_substitute()` to do the actual formatting,
which accepts other value types than `str` and simply calls `__str__()`
on them.

The `safe_substitute()` method is annotated in typeshed with `object`
as the value type for the mapping. However, as `_EngineTemplate` uses
`Any` instead, I went with that.

django-stubs/template/backends/dummy.pyi

@@ -10,4 +10,4 @@ class TemplateStrings(BaseEngine):
 
 class Template(string.Template):
     template: str
-    def render(self, context: dict[str, str] | None = ..., request: HttpRequest | None = ...) -> str: ...
+    def render(self, context: dict[str, Any] | None = ..., request: HttpRequest | None = ...) -> str: ...