Bump urllib3 from 1.26.19 to 1.26.20 in /requirements #15824

Job	Run time
tests (python, 05)	16m 52s
tests (python, 6a)	3m 50s
tests (python, bf)	13m 59s
tests (python-sharded-and-javascript)	16m 27s
	51m 8s

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