use python-is-python3 #626

Job	Run time
docker-master	1h 26m 14s
publish-standalone-dry-run	19s
test-standalone (master, dev, default)	1m 28s
build-js-of-ocaml (master, 4.11.1)	16m 55s
generated-files (master)	22m 53s
build-wasm-of-ocaml (master, 4.14.1)	2h 32m 4s
standalone-haskell (master)	44m 35s
test-amd64	2h 15m 48s
validate-docker-master	3m 25s
deploy-js-wasm-of-ocaml	54s
docker-check-all	7s
	7h 44m 42s

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