Update flake8 requirement from <7.0.0,>=6.0.0 to >=6.0.0,<8.0.0 #9

Job	Run time
build (ubuntu-20.04, 3.9.9, minimal)	43s
build (ubuntu-20.04, 3.9.9, maximal)	41s
test-path-length	10s
build (ubuntu-20.04, 3.10.6, minimal)	47s
build (ubuntu-20.04, 3.10.6, maximal)	39s
test-api (maximal)	1m 4s
test-python (ubuntu-20.04, 3.9.9, minimal)	13s
lint-files (maximal)	21s
test-python (ubuntu-20.04, 3.9.9, maximal)	13s
test-python (ubuntu-20.04, 3.10.6, minimal)	16s
test-yaml (10, 0, minimal)	21s
test-yaml (10, 0, maximal)	16s
test-yaml (10, 1, minimal)	21s
test-python (ubuntu-20.04, 3.10.6, maximal)	15s
test-yaml (10, 1, maximal)	14s
test-yaml (10, 2, minimal)	14s
test-yaml (10, 2, maximal)	14s
test-yaml (10, 3, minimal)	13s
test-yaml (10, 3, maximal)	14s
test-yaml (10, 4, minimal)	15s
test-yaml (10, 4, maximal)	14s
test-yaml (10, 5, minimal)	14s
test-yaml (10, 5, maximal)	14s
test-yaml (10, 6, minimal)	15s
test-yaml (10, 6, maximal)	14s
test-yaml (10, 7, minimal)	21s
test-yaml (10, 7, maximal)	21s
test-yaml (10, 8, minimal)	19s
test-yaml (10, 8, maximal)	20s
test-yaml (10, 9, minimal)	13s
test-yaml (10, 9, maximal)	15s
check-version-and-changelog	0s
check-for-functional-changes	0s
deploy	0s
	10m 44s

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