Honor PEP 621 `requires-python` setting. #4965

Job	Run time
Pre-commit checks / Pre-commit Checks	36s
Check towncrier / Towncrier Checks	23s
Package Briefcase / Create Python Package	24s
Package Automation / Create Python Package	14s
Unit tests (macos-12, 3.9)	1s
Unit tests (macos-12, 3.12)	1s
Unit tests (macos-12, 3.13-dev)	1s
Unit tests (macos-14, 3.9)	1s
Unit tests (macos-14, 3.12)	1s
Unit tests (macos-14, 3.13-dev)	1s
Unit tests (windows-latest, 3.9)	2m 37s
Unit tests (windows-latest, 3.12)	3m 27s
Unit tests (windows-latest, 3.13-dev)	3m 30s
Unit tests (ubuntu-latest, 3.9)	1m 39s
Unit tests (ubuntu-latest, 3.12)	1m 54s
Unit tests (ubuntu-latest, 3.13-dev)	2m 5s
Unit tests (ubuntu-latest, 3.10, false)	1m 32s
Unit tests (ubuntu-latest, 3.11, false)	1m 23s
Project coverage	1s
Verify project	1s
Build app	1s
	19m 53s

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