PEP-8 fix: always compare with None using 'is' or 'is not'.

http://www.python.org/dev/peps/pep-0008/

  Comparisons to singletons like None should always be done with is or
  is not, never the equality operators.

  Also, beware of writing if x when you really mean if x is not None
  -- e.g. when testing whether a variable or argument that defaults to
  None was set to some other value. The other value might have a type
  (such as a container) that could be false in a boolean context!