Improve code readability

[McLaynV]

These changes aim to improve code readablility and maintainability.

My reason for renaming variable id -> tz_id is that it shadows a builtin id.

The others should be clear, but don't hesitate to ask for my reasons for individual changes.

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📚 Documentation preview 📚: https://icalendar--765.org.readthedocs.build/

[coveralls]

Pull Request Test Coverage Report for Build 12727773668

Details

• 26 of 28 (92.86%) changed or added relevant lines in 8 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 96.353%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/icalendar/parser_tools.py	6	7	85.71%
src/icalendar/timezone/equivalent_timezone_ids.py	0	1	0.0%

Totals
Change from base Build 12617583933:	-0.02%
Covered Lines:	4579
Relevant Lines:	4747

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💛 - Coveralls

[niccokunzmann]

Thanks!