docs: remove six from the intersphinx mappings (#17165) #12309

Job	Run time
Test suite with py38-ubuntu, mypyc-compiled	19m 58s
Test suite with py38-windows-64	26m 52s
Test suite with py39-ubuntu	19m 35s
Test mypyc suite with 32-bit Python	8m 22s
Test suite with py310-ubuntu	18m 57s
Test suite with py311-ubuntu, mypyc-compiled	18m 17s
Test suite with py312-ubuntu, mypyc-compiled	20m 33s
mypyc runtime tests with py39-macos	14s
mypyc runtime tests with py38-debug-build-ubuntu	5m 25s
Type check our own code (py38-ubuntu)	1m 47s
Type check our own code (py38-windows-64)	2m 46s
Formatting and code style with Black + ruff	37s
	2h 23m 23s

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