Cleanup rpython/annotator/

Merged Bitbucket Importer requested to merge bitbucket/merged-pr-596 into branch/default

Created originally on Bitbucket by ariava (Arianna Avanzini)

Was already merged in Bitbucket before import, marked as merged by the import user

  • Use 'isinstance()' instead of 'is' in _annotation_key() to account for user-defined subclasses of list and dict.
  • Cleanup assert messages when validating annotations and convert them to exceptions.
  • Remove intermediate variables used just once before return.
  • Cleanup comments.

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