Fix miss-capturing of assign statement after a dedent. closes #11415 This fixes a bug where assign statement were miscaptured when occuring after a dedent. This was due to the fact that : >>> '' in '({[' True That is to say the empty string is in any strings. Add a couple of integration tests and unit tests as well, and also add a warning to public function when not used properly, in particular, check that lines passed to make_tokens_by_line do end with an endline marker (at least for the first line), otherwise the function does not behave properly.