python: avoid equality check for None

PEP8 says that:
"Comparisons to singletons like None should always be done with is or
is not, never the equality operators."

glib uses a mix of "== None" and "is None". This patch changes all
cases to the latter.

gio/gdbus-2.0/codegen/codegen.py

@@ -1068,7 +1068,7 @@ class CodeGenerator:
                            '\n')
 
     def generate_annotations(self, prefix, annotations):
-        if annotations == None:
+        if annotations is None:
             return
 
         n = 0