Update examples: reduce amount of deprecated parameters

[penguinolog]

• Use more constants
• use f-strings
• Fix warnings on mainloop.rst compilation

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 7668336505

• 0 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage remained the same at 71.082%

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Files with Coverage Reduction	New Missed Lines	%
urwid/canvas.py	1	85.75%

Totals
Change from base Build 7667039110:	0.0%
Covered Lines:	8753
Relevant Lines:	12422

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💛 - Coveralls