gha: bump actions/setup-python from 6 to 7 in the github-actions grou… #922

Job	Run time
sync / sync	56s
sync / commit	29s
check / Build docs (latex)	7m 38s
check / Lint translations	54s
check / Build docs (html)	7m 40s
check / Check for zero-width space characters	32s
check / Build docs (epub)	7m 17s
check / Build docs (pdf)	9m 14s
	34m 40s

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