Resolve traitlets type warnings, lint, remove `six` #1630

Job	Run time
jstest/lint (ubuntu, >=16,<17.0.0a0, >=4.0.6,<5.0.0a0, >=4)	5m 24s
build (ubuntu, >=16,<17.0.0a0, >=4.0.0,<5.0.0a0, >=4)	3m 20s
atest ubuntu py3.8 r>=4	4m 20s
smoke source install ubuntu py3.8 lab>=4,<5	1m 47s
atest ubuntu py3.11 r>=4	3m 56s
smoke source install ubuntu py3.11 lab>=4,<5	1m 40s
atest macos py3.8 r>=4	9m 52s
atest macos py3.11 r>=4	10m 24s
atest windows py3.8 r>=4	10m 5s
atest windows py3.11 r>=4	14m 25s
smoke ubuntu py3.8	40s
smoke ubuntu py3.11	45s
smoke ubuntu pypypy-3.8	1m 22s
smoke macos py3.8	1m 26s
smoke macos py3.11	1m 14s
smoke macos pypypy-3.8	1m 13s
smoke windows py3.8	1m 42s
smoke windows py3.11	1m 48s
smoke windows pypypy-3.8	4m 21s
	1h 19m 44s

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