`python` should also be a runtime dependency #481

Job	Run time
run_linter	1m 44s
run_unit_tests (master, with-extras)	2m 14s
run_examples	5m 34s
run_unit_tests (master, no-extras)	1m 57s
run_unit_tests (stable, with-extras)	2m 12s
run_unit_tests (stable, no-extras)	1m 41s
	15m 22s

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