[mypyc] Allow specifying primitives as pure (#17263) #33

Job	Run time
Test mypyc suite with 32-bit Python	7m 17s
Test suite with py38-ubuntu, mypyc-compiled	16m 57s
Test suite with py38-windows-64	19m 40s
Test suite with py39-ubuntu	16m 16s
Test suite with py310-ubuntu	15m 22s
Test suite with py311-ubuntu, mypyc-compiled	15m 17s
Test suite with py312-ubuntu, mypyc-compiled	17m 20s
mypyc runtime tests with py39-macos	5m 1s
mypyc runtime tests with py38-debug-build-ubuntu	4m 47s
Type check our own code (py38-ubuntu)	1m 52s
Type check our own code (py38-windows-64)	2m 41s
Formatting and code style with Black + ruff	36s
	2h 3m 6s

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