Fix ListBox.contents: it should return final entity

[penguinolog]

Fix #146

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

[coveralls]

Pull Request Test Coverage Report for Build 7556787425

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 69.893%

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Totals
Change from base Build 7554908495:	-0.02%
Covered Lines:	8356
Relevant Lines:	12035

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💛 - Coveralls