re-enable the interpretation of the None keyword
introduce maybe_docstr in parse_statement, which should have been introduced way earlier.
implement !=, ==, is, is not operators to work in if statements (they also work in in non if contexts and return a bool value.), includes tests.
Also support the not operator.
Remove the old 'is not' logic to detect if not instances and use them to do branch predictions. This is not necessary anymore, since we now support that in a more general way (flow_analysis).