Honor PEP 621 `requires-python` setting. #4949

Job	Run time
Package Briefcase / Create Python Package	14s
Pre-commit checks / Pre-commit Checks	38s
Check towncrier / Towncrier Checks	22s
Package Automation / Create Python Package	24s
Unit tests (macos-12, 3.9)	1m 44s
Unit tests (macos-12, 3.12)	2m 19s
Unit tests (macos-12, 3.13-dev)	1m 45s
Unit tests (macos-14, 3.9)	1m 24s
Unit tests (macos-14, 3.12)	1m 29s
Unit tests (macos-14, 3.13-dev)	1m 30s
Unit tests (windows-latest, 3.9)	2m 40s
Unit tests (windows-latest, 3.12)	2m 55s
Unit tests (windows-latest, 3.13-dev)	3m 32s
Unit tests (ubuntu-latest, 3.9)	1m 33s
Unit tests (ubuntu-latest, 3.12)	1m 54s
Unit tests (ubuntu-latest, 3.13-dev)	2m 3s
Unit tests (ubuntu-latest, 3.10, false)	1m 34s
Unit tests (ubuntu-latest, 3.11, false)	1m 24s
Project coverage	1s
Build app	1s
Verify project	1s
	29m 27s

Provide feedback