Prefer "is not" over "not ... is". Closes 21540.

pep-0008.txt

@@ -919,6 +919,18 @@ Programming Recommendations
   have a type (such as a container) that could be false in a boolean
   context!
 
+- Use ``is not`` operator rather than ``not ... is``.  While both
+  expressions are functionally identical, the former is more readable
+  and preferred.
+  
+  Yes::
+  
+      if foo is not None:
+      
+  No::
+  
+      if not foo is None:
+
 - When implementing ordering operations with rich comparisons, it is
   best to implement all six operations (``__eq__``, ``__ne__``,
   ``__lt__``, ``__le__``, ``__gt__``, ``__ge__``) rather than relying