Some cleanup in partial plugin (#17423) #437

Job	Run time
Test mypyc suite with 32-bit Python	7m 7s
Test suite with py38-ubuntu, mypyc-compiled	17m 4s
Test suite with py38-windows-64	19m 55s
Test suite with py39-ubuntu	16m 16s
Test suite with py310-ubuntu	15m 31s
Test suite with py311-ubuntu, mypyc-compiled	15m 29s
Test suite with py312-ubuntu, mypyc-compiled	17m 35s
mypyc runtime tests with py39-macos	4m 53s
mypyc runtime tests with py38-debug-build-ubuntu	4m 23s
Type check our own code (py38-ubuntu)	1m 47s
Type check our own code (py38-windows-64)	2m 32s
Formatting and code style with Black + ruff	36s
	2h 3m 8s

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