TreeWidget: do not use deprecated API in update_expanded_icon

[penguinolog]

• Typing: Make Padding and Overlay generic classes
• Extend tests for TreeListBox: cover a deep nested scenario

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 8003697818

Details

• -1 of 14 (92.86%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 72.508%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
urwid/widget/overlay.py	8	9	88.89%

Totals
Change from base Build 8003129794:	0.2%
Covered Lines:	9137
Relevant Lines:	12666

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💛 - Coveralls