use python-is-python3 (#1938) #628

Job	Run time
docker-master	1h 26m 15s
publish-standalone-dry-run	16s
test-standalone (master, dev, default)	1m 24s
build-wasm-of-ocaml (master, 4.14.1)	2h 29m 9s
build-js-of-ocaml (master, 4.11.1)	17m 19s
generated-files (master)	29s
standalone-haskell (master)	47m 17s
validate-docker-master	3m 54s
test-amd64	2h 17m 31s
deploy-js-wasm-of-ocaml	59s
docker-check-all	3s
	7h 24m 36s

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