Honor PEP 621 `requires-python` setting. #4952

Job	Run time
Pre-commit checks / Pre-commit Checks	31s
Package Briefcase / Create Python Package	20s
Package Automation / Create Python Package	22s
Check towncrier / Towncrier Checks	26s
Unit tests (macos-12, 3.9)	2m 1s
Unit tests (macos-12, 3.12)	2m 29s
Unit tests (macos-12, 3.13-dev)	3m 0s
Unit tests (macos-14, 3.9)	1m 8s
Unit tests (macos-14, 3.12)	1m 12s
Unit tests (macos-14, 3.13-dev)	1m 31s
Unit tests (windows-latest, 3.9)	2m 31s
Unit tests (windows-latest, 3.12)	2m 54s
Unit tests (windows-latest, 3.13-dev)	3m 42s
Unit tests (ubuntu-latest, 3.9)	1m 30s
Unit tests (ubuntu-latest, 3.12)	1m 58s
Unit tests (ubuntu-latest, 3.13-dev)	2m 11s
Unit tests (ubuntu-latest, 3.10, false)	1m 30s
Unit tests (ubuntu-latest, 3.11, false)	1m 26s
Project coverage	47s
Build app	0s
Verify project	0s
	31m 29s

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