Fix regression: Overlay not accepted relative positioning

[penguinolog]

Fix: #851

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 8137458297

Details

• 15 of 19 (78.95%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 74.414%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
urwid/widget/overlay.py	15	19	78.95%

Totals
Change from base Build 8136949564:	0.05%
Covered Lines:	9290
Relevant Lines:	12625

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💛 - Coveralls