Fix python 'testing for equality' syntax

'Testing whether an object is 'None' using the == operator is inefficient and potentially incorrect.'  

Per https://lgtm.com/projects/g/KiCad/kicad-source-mirror/?mode=tree&ruleFocus=7900090

eeschema/plugins/python_scripts/kicad_netlist_reader.py

@@ -459,7 +459,7 @@ class netlist():
 
     def addElement(self, name):
         """Add a new kicad generic element to the list"""
-        if self._curr_element == None:
+        if self._curr_element is None:
             self.tree = xmlElement(name)
             self._curr_element = self.tree
         else: