Build flat package for macOS #1722

Job	Run time
CLI responsiveness with latest Python	2m 36s
Downstream builds with Python 3.8	49s
Python 3.8 on Linux	3m 43s
Downstream builds with Python 3.9	48s
Python 3.9 on Linux	4m 57s
Downstream builds with Python 3.10	54s
Python 3.10 on Linux	4m 4s
Downstream builds with Python 3.11	53s
Python 3.11 on Linux	3m 53s
Python 3.8 on Windows	6m 48s
Python 3.9 on Windows	5m 55s
Python 3.10 on Windows	5m 35s
Python 3.11 on Windows	6m 32s
Python 3.8 on macOS	4m 12s
Python 3.9 on macOS	3m 38s
Python 3.10 on macOS	6m 3s
Python 3.11 on macOS	4m 10s
Report coverage	39s
check	3s
	1h 6m 12s

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