[issue2503] Replace "== None/True/False" with "is"

[Amaury Forgeot d'Arc]

Amaury Forgeot d'Arc <amauryfa at gmail.com> added the comment:

Yes, PEP8 says::

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

Reading the patch:
- a change modifies "x == False" into "not x", another moves some lines.
I checked that they are OK (x is already the result of a comparison).
- some occurrences of "x != None" are not replaced. Why? (ex. in
test_ast.py)

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