Improve *HttpResponse.getvalue() types (#422)

* Change returned type of StreamingHttpResponse.getvalue() to bytes

* Add HttpResponse.getvalue() definition

Author: danifus

django-stubs/http/response.pyi

@@ -83,12 +83,13 @@ class HttpResponse(HttpResponseBase):
     context: Context
     resolver_match: ResolverMatch
     def json(self) -> Any: ...
+    def getvalue(self) -> bytes: ...
 
 class StreamingHttpResponse(HttpResponseBase):
     content: Any
     streaming_content: Iterator[Any]
     def __init__(self, streaming_content: Iterable[Any] = ..., *args: Any, **kwargs: Any) -> None: ...
-    def getvalue(self) -> Any: ...
+    def getvalue(self) -> bytes: ...
 
 class FileResponse(StreamingHttpResponse):
     client: Client