Changed arguments list of GraphMLWriterLxml.dump() (#6261)

Changed arguments list of GraphMLWriterLxml.dump to be consistent with the type of GraphMLWriter.dump.
(Otherwise, if we have some GraphMLWriterLxml A, and some user calls A.dump(stream) (maybe in the context of a function which takes a GraphMLWriter) then it will call the method of the parent class rather than that of the child class.)
This also makes static analysis tools happier, which reduces the amount of things to look at when debugging.

networkx/readwrite/graphml.py

@@ -826,7 +826,7 @@ def add_attributes(self, scope, xml_obj, data, default):
     def __str__(self):
         return object.__str__(self)
 
-    def dump(self):
+    def dump(self, stream=None):
         self._graphml.__exit__(None, None, None)
         self._xml_base.__exit__(None, None, None)