Bump flask in /circleci/images/citusupgradetester/files/etc #500

Job	Run time
prepare_pgversion_matrix	4s
push-test-images-for-dev (stylechecker)	4m 3s
push-test-images-for-dev (citusupgradetester)	1m 17s
push-test-images-for-dev (pgupgradetester)	2m 11s
push-shared-images-for-release	0s
push-test-images-for-dev (extbuilder, 17.9)	1m 23s
push-test-images-for-dev (extbuilder, 18.3)	1m 22s
push-test-images-for-dev (extbuilder, 16.13)	1m 28s
push-test-images-for-dev (failtester, 18.3)	1m 56s
push-test-images-for-dev (exttester, 16.13)	4m 53s
push-test-images-for-dev (failtester, 16.13)	1m 53s
push-test-images-for-dev (exttester, 17.9)	5m 7s
push-test-images-for-dev (exttester, 18.3)	5m 57s
push-test-images-for-dev (failtester, 17.9)	3m 57s
push-test-images-for-release	-1s
	35m 30s

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