[mypyc] Allow specifying primitives as pure #12407

Job	Run time
Test mypyc suite with 32-bit Python	7m 11s
Test suite with py38-ubuntu, mypyc-compiled	17m 11s
Test suite with py38-windows-64	19m 18s
Test suite with py39-ubuntu	16m 6s
Test suite with py310-ubuntu	15m 20s
Test suite with py311-ubuntu, mypyc-compiled	14m 33s
Test suite with py312-ubuntu, mypyc-compiled	16m 38s
mypyc runtime tests with py39-macos	4m 28s
mypyc runtime tests with py38-debug-build-ubuntu	4m 26s
Type check our own code (py38-ubuntu)	1m 46s
Type check our own code (py38-windows-64)	2m 50s
Formatting and code style with Black + ruff	37s
	2h 0m 24s

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