Fix deprecated _set_focus method usage

[penguinolog]

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)

Fix #658

[coveralls]

Pull Request Test Coverage Report for Build 6574691143

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 74.388%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
urwid/monitored_list.py	0	1	0.0%

Totals
Change from base Build 6574657364:	0.0%
Covered Lines:	7519
Relevant Lines:	10291

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💛 - Coveralls