Bump minimum required Python version to 3.9 #181

Job	Run time
publish / Load build targets	10s
publish / Build source distribution	40s
publish / Build cp*-manylinux_x86_64 wheels	1m 56s
publish / Build cp*-manylinux_aarch64 wheels	2m 18s
publish / Build pp39-manylinux_x86_64 wheels	2m 51s
publish / Build cp*-macosx_x86_64 wheels	9m 25s
publish / Build cp*-macosx_arm64 wheels	56s
publish / Build pp39-macosx_x86_64 wheels	8m 18s
publish / Build cp*-win32 wheels	4m 45s
publish / Build cp*-win_amd64 wheels	4m 31s
publish / Build pp39-win_amd64 wheels	5m 14s
publish / Upload	0s
	41m 4s

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