Annotate nbytes in update_data as a dict

The one case where this variable is used it is a `dict`. So go ahead and
annotate it that way. Should speed up usage of this variable when
cythonized.

distributed/scheduler.py

@@ -5566,7 +5566,12 @@ def add_keys(self, comm=None, worker=None, keys=()):
         return "OK"
 
     def update_data(
-        self, comm=None, who_has=None, nbytes=None, client=None, serializers=None
+        self,
+        comm=None,
+        who_has=None,
+        nbytes: dict = None,
+        client=None,
+        serializers=None,
     ):
         """
         Learn that new data has entered the network from an external source