ci: Bump Emacs 29.x to 3 #18

Job	Run time
test (ubuntu-latest, 26.3, false)	34s
test (ubuntu-latest, 27.2, false)	28s
test (ubuntu-latest, 28.2, false)	31s
test (ubuntu-latest, 29.3, false)	41s
test (macos-latest, 26.3, false)	1m 36s
test (macos-latest, 27.2, false)	1m 48s
test (macos-latest, 28.2, false)	1m 45s
test (macos-latest, 29.3, false)	1m 59s
test (windows-latest, 26.3, false)	1m 15s
test (windows-latest, 27.2, false)	1m 45s
test (windows-latest, 28.2, false)	46s
test (windows-latest, 29.3, false)	44s
test (ubuntu-latest, snapshot, true)	25s
test (macos-latest, snapshot, true)	1m 31s
test (windows-latest, snapshot, true)	1m 10s
	16m 58s

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